IceCream-Cpp is a little (single header) library to help with the print debugging in C++11 and forward.
Contents
- Install
- Usage
- Pitfalls
With IceCream, an execution inspection:
auto my_function(int i, double d) -> void
{
std::cout << "1" << std::endl;
if (condition)
std::cout << "2" << std::endl;
else
std::cout << "3" << std::endl;
}can be coded instead:
auto my_function(int i, double d) -> void
{
IC();
if (condition)
IC();
else
IC();
}and will print something like:
ic| test.cpp:34 in "void my_function(int, double)"
ic| test.cpp:36 in "void my_function(int, double)"
Also, any variable inspection like:
std::cout << "a: " << a
<< ", b: " << b
<< ", sum(a, b): " << sum(a, b)
<< std::endl;can be simplified to:
IC(a, b, sum(a, b));and will print:
ic| a: 7, b: 2, sum(a, b): 9
We also can inspect the data flowing through a range views pipeline (both STL
ranges and
Range-v3), by inserting a IC_V() function at the
point of interest:
auto rv = std::vector<int>{1, 0, 2, 3, 0, 4, 5}
| vws::split(0)
| IC_V()
| vws::enumerate;So that when we iterate on rv, we will see the printing:
ic| range_view_63:16[0]: [1]
ic| range_view_63:16[1]: [2, 3]
ic| range_view_63:16[2]: [4, 5]
This library is inspired by the original Python IceCream library.
The IceCream-Cpp is a one file, header only library, having the STL as its only
dependency. The most immediate way to use it is just copy the icecream.hpp header into
your project.
To properly install it system wide, together with the CMake project files, run these commands in IceCream-Cpp project root directory:
mkdir build
cd build
cmake ..
cmake --install .If using Nix, IceCream-Cpp can be included as a flakes input as
inputs.icecream-cpp.url = "github:renatoGarcia/icecream-cpp";The IceCream-Cpp flake defines an overlay, so that it can be used when importing nixpkgs:
import nixpkgs {
system = "x86_64-linux";
overlays = [
icecream-cpp.overlays.default
];
}Doing this, an icecream-cpp derivation will be added to the nixpkgs attribute set.
A working example of how to use IceCream-Cpp in a flake project is here.
The released versions are available on Conan too:
conan install icecream-cpp/0.3.1@If using CMake:
find_package(IcecreamCpp)
include_directories(${IcecreamCpp_INCLUDE_DIRS})will add the installed directory within the include paths list.
After including the icecream.hpp header in a source file:
#include <icecream.hpp>all the functionalities of IceCream-Cpp library will be available by the functions
IC, IC_A, and
IC_V; together with its respective counterparts IC_F,
IC_FA, and IC_FV; that behave the same but accept an output formatting
string as its first argument.
The IC is the simplest of the IceCream functions. If called with no arguments it will
print the prefix, the source file name, the current line number, and the
current function signature. The code:
auto my_function(int foo, double bar) -> void
{
// ...
IC();
// ...
}will print:
ic| test.cpp:34 in "void my_function(int, double)"
If called with arguments it will print the prefix, those arguments names, and its values. The code:
auto v0 = std::vector<int>{1, 2, 3};
auto s0 = std::string{"bla"};
IC(v0, s0, 3.14);will print:
ic| v0: [1, 2, 3], s0: "bla", 3.14: 3.14
The variant IC_F behaves the same as the IC function, but accepts an output
formatting string as its first argument.
To print the data flowing through a range views pipeline (both STL
ranges and
Range-v3), we use the IC_V function, which
will print any input it receives from the previous view. Since the IC_V function is
within a range views pipeline, the printing will be done lazily, while each element is
generated. For instance:
namespace vws = std::views;
auto v0 = vws::iota('a') | vws::enumerate | IC_V() | vws::take(3);
for (auto e : v0)
{
//...
}In this code nothing will be printed when v0 is created, just when iterating over it. At
each iteration in the for loop one line will be printed, until we have the output:
ic| range_view_61:53[0]: (0, 'a')
ic| range_view_61:53[1]: (1, 'b')
ic| range_view_61:53[2]: (2, 'c')
Note
IceCream-Cpp will try to detect if the Range-v3 library is installed, and if so, the
support to it will be automatically enabled. When using C++11 and C++14 however, there
is a chance of having Range-v3 in the system, but IceCream not finding it. To make sure
that the support to Range-v3 is enabled, just define the macro ICECREAM_RANGE_V3
before including the icecream.hpp header
The IC_V function has two optional parameters, IC_V(name, projection).
The variable name used to the view when printing. The printing layout is: <name>[<idx>]: <value>. If the name parameter is not used, the default value to <name> is
range_view_<source_location>.
The code:
vws::iota('a') | vws::enumerate | IC_V("foo") | vws::take(2);when iterated over will print:
ic| foo[0]: (0, 'a')
ic| foo[1]: (1, 'b')
A callable that will receive as input the elements from the previous view and must return the actual object to be printed.
The code:
vws::iota('a') | vws::enumerate | IC_V([](auto e){return std::get<1>(e);}) | vws::take(2);when iterated over will print:
ic| range_view_61:53[0]: 'a'
ic| range_view_61:53[1]: 'b'
Note
The IC_V function will still forward to the next view an unchanged input element,
exactly as it was received from the previous view. None action done by the projection
function will have any effect on that.
The variant IC_FV has the same behavior as the IC_V function, but accepts an output
formatting string as its first argument.
Except when called with exactly one argument, the IC function will
return a tuple with all its input arguments. If called with one argument it will return
the argument itself.
This is done this way so that you can use IC to inspect a function argument at calling
point, with no further code change. In the code:
my_function(IC(MyClass{}));the MyClass object will be forwarded to my_function exactly the same as if the
IC function was not there. The my_function will continue receiving a rvalue reference to a
MyClass object.
This approach however is not so practical when the function has many arguments. On the code:
my_function(IC(a), IC(b), IC(c), IC(d));besides writing four times the IC function, the printed output will be split in four
distinct lines. Something like:
ic| a: 1
ic| b: 2
ic| c: 3
ic| d: 4
Unfortunately, just wrapping all the four arguments in a single IC call will not work
too. The returned value will be a std:::tuple with (a, b, c, d) and the my_function
expects four arguments.
To work around that, there is the IC_A function. IC_A behaves exactly like the IC
function, but receives a callable
as its first argument, and will call it using all the next arguments, printing all of them
before that. That previous example code could be rewritten as:
IC_A(my_function, a, b, c, d);and this time it will print:
ic| a: 1, b: 2, c: 3, d: 4
The IC_A function will return the same value as returned by the callable. The code:
auto mc = std::make_unique<MyClass>();
auto r = IC_A(mc->my_function, a, b);behaves exactly the same as:
auto mc = std::make_unique<MyClass>();
auto r = mc->my_function(a, b);but will print the values of a and b.
The variant IC_FA behaves the same as the IC_A function, but accepts an output
formatting string as its first argument, even before the callable
argument.
It is possible to configure how the value must be formatted while printing. The following code:
auto a = int{42};
auto b = int{20};
IC_F("#X", a, b);will print:
ic| a: 0X2A, b: 0X14
when using the IC_F variant instead of the plain IC functio. A
similar result would be obtained if using IC_FA and IC_FV in place of
IC_A and IC_V
respectively.
When using the formatting function variants (IC_F and IC_FA), the same formatting
string will be applied by default to all the arguments. That could be a problem if we wish
to have arguments with distinct formatting, or if the arguments have multiple types with
non mutually valid syntaxes. Therefore, to set a distinct formatting string to a specific
argument we can wrap it with the IC_ function. The code:
auto a = int{42};
auto b = int{20};
IC_F("#X", a, IC_("d", b));will print:
ic| a: 0X2A, b: 20
The IC_ function can be used within the plain IC (or IC_A) function too:
auto a = int{42};
auto b = int{20};
IC(IC_("#x", a), b);will print:
ic| a: 0x2a, b: 20
The last argument in an IC_ function call is the one that will be printed, all other
arguments that come before the last will be converted to a string using the
to_string function
and concatenated as the resulting formatting string.
auto a = float{1.234};
auto width = int{7};
IC(IC_("*<",width,".3", a));Will have as result a formatting string "*<7.3", and will print:
ic| a: 1.23***
Just for completeness in the examples, an usage of IC_FA and IC_FV would be:
IC_FA("#x", my_function, 10, 20);
auto rv0 = vws::iota(0) | IC_FV("[::2]:#x", "bar") | vws::take(5);This will print:
ic| 10: 0xa, 20: 0x14
and when iterating on rv0:
ic| bar[0]: 0
ic| bar[2]: 0x2
ic| bar[4]: 0x4
To IC_F and IC_FA, the syntax specification of the formatting strings depends both on
the type T being printed, and in that type's printing strategy
used by IceCream.
To IC_FV, the formatting syntax if the same as the Range format
string.
Character encoding in C++ is messy.
The char8_t, char16_t, and char32_t strings are well defined. They are capable, and
do hold Unicode code units of 8, 16, and 32 bits respectively, and they are encoded in
UTF-8, UTF-16, and UTF-32 also respectively.
The char strings have a well defined code unit bit size (given by
CHAR_BIT, usually 8 bits), but there
are no requirements about its encoding.
The wchar_t strings have neither a well defined code unit
size, nor any
requirements about its encoding.
In a code like this:
auto const str = std::string{"foo"};
std::cout << str;We will have three character encoding points of interest. In the first one, before
compiling, that code will be in a source file in an unspecified "source encoding". In the
second interest point, the compiled binary will have the "foo" string saved in an
unspecified "execution encoding". Finally on the third point, the "foo" byte stream
received by std::cout will be ultimately forwarded to the system, that expects the
stream being encoded in an also unspecified "output encoding".
From that three interest points of character encoding, both "execution encoding", and "output encoding" have impact in the inner working of Icecream-cpp, and there is no way to know for sure what is the used encoding in both of them. In face of this uncertainty, the adopted strategy is offer a reasonable default transcoding function, that will try convert the data to the right encoding, and allow the user to use its own implementation when needed.
Except for wide and Unicode string types (discussed below), when printing any other type we will have its serialized textual data in "execution encoding". That "execution encoding" may or may not be the same as the "output encoding", this one being the encoding expected by the configured output. Because of that, before we send that data to the output, we must transcode it to make sure that we have it in "output encoding". To that end, before delivering the text data to the output, we send it to the configured output_transcoder function, that must ensure it is encoded in the correct "output encoding".
When printing the wide and Unicode string types, we need to have one more transcoding level, because it is possible that the text data is in a distinct character encoding from the expected "execution encoding". Because of that, additional logic is applied to make sure that the strings are in "execution encoding" before we send them to output. This is further discussed in wide strings, and unicode strings sections.
The Icecream-cpp configuration system works "layered by scope". At the basis level we have
the global IC_CONFIG object. That global instance is shared by the whole running
program, as would be expected of a global variable. It is created with all config options
at its default values, and any change is readily seen by the whole program.
At any point of the code we can create a new config layer at the current scope by
instantiating a new IC_CONFIG variable, calling the IC_CONFIG_SCOPE() macro. All the
config options of this new instance will be in an "unset" state by default, and any
request to an option value not yet set will be delegated to its parent. That request will
go up on the parent chain until the first one having that option set answers.
All config options are set by using accessor methods of the IC_CONFIG object, and they
can be chained:
IC_CONFIG
.prefix("ic: ")
.show_c_string(false)
.line_wrap_width(70);IC_CONFIG is just a regular variable with a funny name to make a collision extremely
unlikely. When calling any IC*(...) macro, it will pick the IC_CONFIG instance at
scope by doing an unqualified name
lookup, using the same
rules applied to any other regular variable.
To summarize all the above, in the code:
auto my_function() -> void
{
IC_CONFIG.line_wrap_width(20);
IC_CONFIG_SCOPE();
IC_CONFIG.context_delimiter("|");
IC_CONFIG.show_c_string(true);
{
IC_CONFIG_SCOPE();
IC_CONFIG.show_c_string(false);
// A
}
// B
}At line A, the value of IC_CONFIG's line_wrap_width, context_delimiter, and
show_c_string will be respectively: 20, "|", and false.
After the closing of the innermost scope block, at line B, the value of IC_CONFIG's
line_wrap_width, context_delimiter, and show_c_string will be respectively: 20,
"|", and true.
The reading and writing operations on IC_CONFIG objects are thread safe.
Note
Any modification in an IC_CONFIG, other than to the global instance, will be seen only
within the current scope. As consequence, those modifications won't propagate to the
scope of any called function.
Enable or disable the output of IC(...) macro, enabled default.
- get:
auto is_enabled() const -> bool;
- set:
auto enable() -> Config&; auto disable() -> Config&;
The code:
IC(1);
IC_CONFIG.disable();
IC(2);
IC_CONFIG.enable();
IC(3);will print:
ic| 1: 1
ic| 3: 3
Sets where the serialized textual data will be printed. By default that data will be
printed on the standard error output, the same as std::cerr.
- get:
auto output() const -> std::function<void(std::string const&)>;
- set:
template <typename T> auto output(T&& t) -> Config&;
Where the type T can be any of:
- A class inheriting from
std::ostream. - A class having a method
push_back(char). - An output iterator that accepts the operation
*it = 'c'
For instance, the code:
auto str = std::string{};
IC_CONFIG.output(str);
IC(1, 2);Will print the output "ic| 1: 1, 2: 2\n" on the str string.
Warning
Icecream-cpp won't take ownership of the argument t, so care must be taken by the user
to ensure that it is alive.
A function that generate the text that will be printed before each output.
- get:
auto prefix() const -> std::function<std::string()>;
- set:
template <typename... Ts> auto prefix(Ts&& ...values) -> Config&;
Where the types Ts can be any of:
- A string,
- A callable
T() -> U, whereUhas an overload ofoperator<<(ostream&, U).
The printed prefix will be a concatenation of all those elements.
The code:
IC_CONFIG.prefix("icecream| ");
IC(1);
IC_CONFIG.prefix([]{return 42;}, "- ");
IC(2);
IC_CONFIG.prefix("thread ", std::this_thread::get_id, " | ");
IC(3);will print:
icecream| 1: 1
42- 2: 2
thread 1 | 3: 3
Controls if a char* variable should be interpreted as a null-terminated C string
(true) or a pointer to a char (false). The default value is true.
- get:
auto show_c_string() const -> bool;
- set:
auto show_c_string(bool value) -> Config&;
The code:
char const* flavor = "mango";
IC_CONFIG.show_c_string(true);
IC(flavor);
IC_CONFIG.show_c_string(false);
IC(flavor);will print:
ic| flavor: "mango";
ic| flavor: 0x55587b6f5410
Function that transcodes a wchar_t string, from a system defined encoding to a char
string in the system "execution encoding".
- get:
auto wide_string_transcoder() const -> std::function<std::string(wchar_t const*, std::size_t)>;
- set:
auto wide_string_transcoder(std::function<std::string(wchar_t const*, std::size_t)> transcoder) -> Config&; auto wide_string_transcoder(std::function<std::string(std::wstring_view)> transcoder) -> Config&;
There is no guarantee that the input string will end on a null terminator (this is the actual semantic of string_view), so the user must observe the input string size value.
The default implementation will check if the C locale is set to other value than "C" or
"POSIX". If yes, it will forward the input to the
std::wcrtomb function.
Otherwise, it will assume that the input is Unicode encoded (UTF-16 or UTF-32, accordingly
to the byte size of wchar_t), and transcoded it to UTF-8.
Function that transcodes a char32_t string, from a UTF-32 encoding to a char string in
the system "execution encoding".
- get:
auto unicode_transcoder() const -> std::function<std::string(char32_t const*, std::size_t)>;
- set:
auto unicode_transcoder(std::function<std::string(char32_t const*, std::size_t)> transcoder) -> Config&; auto unicode_transcoder(std::function<std::string(std::u32string_view)> transcoder) -> Config&;
There is no guarantee that the input string will end on a null terminator (this is the actual semantic of string_view), so the user must observe the input string size value.
The default implementation will check the C locale is set to other value than "C" or "POSIX". If yes, it will forward the input to the std::c32rtomb function. Otherwise, it will just transcoded it to UTF-8.
This function will be used to transcode all the char8_t, char16_t, and char32_t
strings. When transcoding char8_t and char16_t strings, they will be first converted
to a char32_t string, before being sent as input to this function.
Function that transcodes a char string, from the system "execution encoding" to a char
string in the system "output encoding", as expected by the configured output.
- get:
auto output_transcoder() const -> std::function<std::string(char const*, std::size_t)>;
- set:
auto output_transcoder(std::function<std::string(char const*, std::size_t)> transcoder) -> Config&; auto output_transcoder(std::function<std::string(std::string_view)> transcoder) -> Config&;
There is no guarantee that the input string will end on a null terminator (this is the actual semantic of string_view), so the user must observe the input string size value.
The default implementation assumes that the "execution encoding" is the same as the "output encoding", and will just return an unchanged input.
The maximum number of characters before the output be broken on multiple lines. Default
value of 70.
- get:
auto line_wrap_width() const -> std::size_t;
- set:
auto line_wrap_width(std::size_t value) -> Config&;
If the context (source name, line number, and function name) should be printed even when
printing variables. Default value is false.
- get:
auto include_context() const -> bool;
- set:
auto include_context(bool value) -> Config&;
The string separating the context text from the variables values. Default value is "- ".
- get:
auto context_delimiter() const -> std::string;
- set:
auto context_delimiter(std::string const& value) -> Config&;
In order to be printable, a type T must satisfy one of the strategies described in the
next sections. If it happens that multiple strategies are satisfied, the one with the
higher precedence will be chosen.
The strategy with the highest precedence is to use the STL stream-based
I/O. Consequently, when printing an object of type
T, if there exist an overloaded function operator<<(ostream&, T), it will be used.
C strings are ambiguous. Should a char* foo variable be interpreted as a pointer to a
single char or as a null-terminated string? Likewise, is the char bar[] variable an
array of single characters or a null-terminated string? Is char baz[3] an array with
three single characters or is it a string of size two plus a '\0'?
Each one of those interpretations of foo, bar, and baz would be printed in a
distinct way. To the code:
char flavor[] = "pistachio";
IC(flavor);all three outputs below are correct, each one having a distinct interpretation of what
should be the flavor variable.
ic| flavor: 0x55587b6f5410
ic| flavor: ['p', 'i', 's', 't', 'a', 'c', 'h', 'i', 'o', '\0']
ic| flavor: "pistachio"
The IceCream-Cpp policy is handle any bounded char array (i.e.: array with a known size)
as an array of single characters. So the code:
char flavor[] = "chocolate";
IC(flavor);will print:
ic| flavor: ['c', 'h', 'o', 'c', 'o', 'l', 'a', 't', 'e', '\0']
unbounded char[] arrays (i.e.: array with an unknown size) will decay to char*
pointers, and will be printed either as a string or a pointer as configured by the
show_c_string option.
Any realization of wchar_t strings, like wchar_t*, std::wstring, and std::string_view
Since the output expects a char string, we must convert the text data to that
format, making sure that it is in "execution encoding". Icecream-cpp implements a default
transcoder function for doing that, but is possible to customize it by setting the
wide_string_transcoder option.
Any realization of char8_t, char16_t, and char32_t strings, like char32_t*,
std::u8string, and std::u16string_view
Since the output expects a char string, we must convert the data to that
format, making sure that it is in "execution encoding". Icecream-cpp implements a default
transcoder function for doing that, but is possible to customize it by setting the
unicode_transcoder option.
A type T is streamable if an overloading function operator<<(std::ostream&, T)is
defined. This is the printing strategy with the highest priority, so if that overload
exists to a type T, it will be the used strategy.
The STL stream-based I/O hasn't the concept of a formatting string, all the configurations are done using manipulators. Because of that, we have created a custom formatting string syntax, strongly based on {fmt} and STL Formatting.
It has the following specification:
format_spec ::= [[fill]align][sign]["#"][width]["." precision][type]
fill ::= <a character>
align ::= "<" | ">" | "v"
sign ::= "+" | "-"
width ::= integer
precision ::= integer
type ::= "a" | "A" | "d" | "e" | "E" | "f" | "F" | "g" | "G" | "o" | "x" | "X"
integer ::= digit+
digit ::= "0"..."9"
The fill character can be any char. The presence of a fill character is signaled by the character following it, which must be one of the alignment options. The meaning of the alignment options is as follows:
| Symbol | Meaning |
|---|---|
'<' |
Left align within the available space. |
'>' |
Right align within the available space. This is the default. |
'v' |
Internally align the data, with the fill character being placed between the digits and either the base or sign. Applies to integer and floating-point. |
Note that unless a minimum field width is defined, the field width will always be the same size as the data to fill it, so that the alignment option has no meaning in this case.
The sign option is only valid for number types, and can be one of the following:
| Symbol | Meaning |
|---|---|
'+' |
A sign will be used for both nonnegative as well as negative numbers. |
'-' |
A sign will be used only for negative numbers. This is the default. |
Causes the “alternate form” to be used for the conversion. The alternate form is defined differently for different types. This option is only valid for integer and floating-point types. For integers, when binary, octal, or hexadecimal output is used, this option adds the prefix respective "0b" ("0B"), "0", or "0x" ("0X") to the output value. Whether the prefix is lower-case or upper-case is determined by the case of the type specifier, for example, the prefix "0x" is used for the type 'x' and "0X" is used for 'X'. For floating-point numbers the alternate form causes the result of the conversion to always contain a decimal-point character, even if no digits follow it. Normally, a decimal-point character appears in the result of these conversions only if a digit follows it. In addition, for 'g' and 'G' conversions, trailing zeros are not removed from the result.
A decimal integer defining the minimum field width. If not specified, then the field width will be determined by the content.
The precision is a decimal number indicating how many digits should be displayed after the decimal point for a floating-point value formatted with 'f' and 'F', or before and after the decimal point for a floating-point value formatted with 'g' or 'G'. For non-number types the field indicates the maximum field size - in other words, how many characters will be used from the field content. The precision is not allowed for integer, character, Boolean, and pointer values. Note that a C string must be null-terminated even if precision is specified.
Determines how the data should be presented.
The available integer presentation types are:
| Symbol | Meaning |
|---|---|
'd' |
Decimal integer. Outputs the number in base 10. |
'o' |
Octal format. Outputs the number in base 8. |
'x' |
Hex format. Outputs the number in base 16, using lower-case letters for the digits above 9. Using the '#' option with this type adds the prefix "0x" to the output value. |
'X' |
Hex format. Outputs the number in base 16, using upper-case letters for the digits above 9. Using the '#' option with this type adds the prefix "0X" to the output value. |
The available presentation types for floating-point values are:
| Symbol | Meaning |
|---|---|
'a' |
Hexadecimal floating point format. Prints the number in base 16 with prefix "0x" and lower-case letters for digits above 9. Uses 'p' to indicate the exponent. |
'A' |
Same as 'a' except it uses upper-case letters for the prefix, digits above 9 and to indicate the exponent. |
'e' |
Exponent notation. Prints the number in scientific notation using the letter ‘e’ to indicate the exponent. |
'E' |
Exponent notation. Same as 'e' except it uses an upper-case 'E' as the separator character. |
'f' |
Fixed point. Displays the number as a fixed-point number. |
'F' |
Fixed point. Same as 'f', but converts nan to NAN and inf to INF. |
'g' |
General format. For a given precision p >= 1, this rounds the number to p significant digits and then formats the result in either fixed-point format or in scientific notation, depending on its magnitude. A precision of 0 is treated as equivalent to a precision of 1. |
'G' |
General format. Same as 'g' except switches to 'E' if the number gets too large. The representations of infinity and NaN are uppercased, too. |
The std::unique_ptr<T> (before C++20) and boost::scoped_ptr<T> types will be printed
like usual raw pointers.
The std::weak_ptr<T> and boost::weak_ptr<T> types will print their address if they are
valid or "expired" otherwise. The code:
auto v0 = std::make_shared<int>(7);
auto v1 = std::weak_ptr<int> {v0};
IC(v1);
v0.reset();
IC(v1);will print:
ic| v1: 0x55bcbd840ec0
ic| v1: expired
A range is any type able to provide a [begin, end) iterator pair. In precise terms,
the Icecream-cpp library is able to print a range type R if it fulfills the
forward_range concept. In
roughly terms, a range type R having an iterator type I and a sentinel type S (used
to mark the end of the range, can be the same type as I) is a forward range if all the
following operations are valid:
I i0 = begin(r);
S s = end(r);
I i1(i0);
i0 == i1
i0 != i1
i0 == s
i0 != s
++i0;
*i0;If all that operations are valid, the type R is not streamable but
the elements inside R are, IceCream will print all items within R in place of R
itself.
The code:
auto v0 = std::list<int>{10, 20, 30};
IC(v0);will print:
ic| v0: [10, 20, 30]
Although lazily printing one item at a time, instead of the whole range at once. The
IC_V function is printing a range too.
The accepted formatting string to a range type is a combination of both a range formatting and its elements formatting. The range formatting is syntactically and semantically almost identical to the Python slicing.
Formally, the accepted range types formatting string is:
format_spec ::= [range_fmt][":"elements_fmt]
range_fmt ::= "[" slicing | index "]"
slicing ::= [lower_bound] ":" [upper_bound] [ ":" [stride] ]
lower_bound ::= integer
upper_bound ::= integer
stride ::= integer
index ::= integer
integer ::= ["-"]digit+
digit ::= "0"..."9"
The same elements_fmt string will be used by all the printing elements, so it will have
the same syntax as the formatting string of the range elements.
The code:
auto arr = std::vector<int>{10, 11, 12, 13, 14, 15};
IC_F("[:2:-1]:#x", arr);will print:
ic| arr: [:2:-1]->[0xf, 0xe, 0xd]
Even though the specification says that lower_bound, upper_bound, stride, and
index, can have any integer value, some range capabilities can restrict them to just
positive values.
If a range is not sized, the
lower_bound, upper_bound, and index values must be positive. Similarly, if a range
is not bidirectional the
stride value must be positive too.
When printing within a range views pipeline using the IC_FV
function, all the lower_bound, upper_bound, and index values must be positive.
A std::pair<T1, T2> or std::tuple<Ts...> typed variables will print all of its
elements.
The code:
auto v0 = std::make_pair(10, 3.14);
auto v1 = std::make_tuple(7, 6.28, "bla");
IC(v0, v1);will print:
ic| v0: (10, 3.14), v1: (7, 6.28, "bla")
The tuple like formatting specification is based on the syntax suggested in the Formatting Ranges paper. Since that part hasn't done to the proposal, with any future change we may revisit it here too.
tuple_spec ::= [casing][content]
casing ::= "n" | "m"
content ::= (delimiter element_fmt){N}
delimiter ::= <a character, the same to all N expansions>
element_fmt ::= <format specification of the element type>
Where the number N of repetitions in content rule is the tuple size.
The code:
auto v0 = std::make_tuple(20, "foo", 0.0123);
IC_F("n|#x||.3e", v0);will print:
ic| ic| v0: 0x14, "foo", 1.230e-02
Controls the tuple enclosing characters and separator. If not used, the default behavior
is to enclose the values between "(" and ")", and separated by ", ".
If n is used, the tuple won't be enclosed by parentheses. If m is used the tuple will
be printed "map value like", i.e.: not enclosed by parentheses and using ": " as
separator. The m specifier is valid just to a pair or 2-tuple.
The formatting string of each tuple element, separated by a delimiter character. This
can be any character, which value will be defined by the fist read char when parsing
this rule.
Each element_fmt string will be forwarded to the respective tuple element when printing
it, so it must follow the formatting specification of that particular element type.
A std::optional<T> typed variable will print its value, if it has one, or nullopt
otherwise.
The code:
auto v0 = std::optional<int> {10};
auto v1 = std::optional<int> {};
IC(v0, v1);will print:
ic| v0: 10, v1: nullopt
A std::variant<Ts...> or boost::variant2::variant<Ts...> typed variable will print its
value.
The code:
auto v0 = std::variant<int, double, char> {4.2};
IC(v0);will print:
ic| v0: 4.2
Types inheriting from std::exception will print the return of std::exception::what()
method. If beyond that it inherits from boost::exception too, the response of
boost::diagnostic_information() will be also printed.
The code:
auto v0 = std::runtime_error("error description");
IC(v0);will print:
ic| v0: error description
If using Clang >= 15, a class will be printable even without an operator<<(ostream&, T) overload.
The code:
class S
{
public:
float f;
int ii[3];
};
S s = {3.14, {1,2,3}};
IC(s);will print:
ic| s: {f: 3.14, ii: [1, 2, 3]}
IC(...) is a preprocessor macro, it can cause conflicts if there is some
other IC identifier on code. To change the IC(...) macro to a longer ICECREAM(...)
one, just define ICECREAM_LONG_NAME before the inclusion of icecream.hpp header:
#define ICECREAM_LONG_NAME
#include "icecream.hpp"While most compilers will work just fine, until the C++20 the standard requires at least
one argument when calling a variadic macro. To handle this the nullary macros IC0() and
ICECREAM0() are defined alongside IC(...) and ICECREAM(...).