InputText CallbackResize does not work with EnterReturnsTrue if user code doesn't persist their string

[PeterJohnson]

Version/Branch of Dear ImGui:

Version: 1.72b (17202)
Branch: master

Back-end/Renderer/Compiler/OS

Back-ends: imgui_impl_opengl3.cpp + imgui_impl_glfw.cpp
Operating System: Linux

My Issue:

Using ImGuiInputTextFlags_CallbackResize (e.g. as used in imgui_stdlib for outputting to std::string) does not interoperate correctly with the ImGuiInputTextFlags_EnterReturnsTrue flag. When the EnterReturnsTrue flag is specified, the returned string data is correct, but the length is incorrect--it's the original length instead of the new length (if longer), and the original length-1 (if shorter).

Standalone, minimal, complete and verifiable example:

#include "imgui_stdlib.h"
ImGui::Begin("Example Bug");
std::string myInput = "abcd";
if (ImGui::InputText("bug", &myInput, ImGuiInputTextFlags_EnterReturnsTrue))
  std::cout << "result: '" << myInput << "'\n";
ImGui::End();

Edit the string to make it shorter or longer and hit Enter. If the input text is longer (e.g. "hello" is input), the result will be truncated at length=4 (e.g. "hell"). If the input text is shorter, the result will be length=3 and contain a mix of input text and original text depending on the input text length.

[rokups]

I am unable to reproduce the issue. Could you please test latest version from here? Also please clarify what field of ImGuiInputTextCallbackData you refer to by saying "size" "length"? data->BufTextLen is always equal to length of entered string.

[ocornut]

I can repro this and as mentioned the string data is correct but std::string Size member isn't:

printf("1 result: '%s'\n", myInput.c_str());
std::cout << "2 result: '" << myInput << "'\n";

1 result: 'hello'
2 result: 'hell'

EDIT some/all std::string implementation maintain Size which I presume is relied on by the << operator instead of the null terminator, so using printf you wouldn't notice the bug right away.

[ocornut]

Note that the bug doesn't happen if you actually preserve the input buffer as you are generally meant to, e.g.
std::string myInput = "abcd";

InputText is specific because it owns a copy of the data, it allows running without persisting user storage, but few people use that technique and this explain why nobody reported this issue before.

[ocornut]

This is now fixed, however:

Using CallbackResize along with no persisting user storage (as implied by your repro) will trigger a likely allocation every single frame when editing the string. This is not ideal.

It's actually surprising that your repro doesn't save the string accross frames and I was wondering if it was how your real code was actually laid out? If it was NOT setup like that (= you actually persisted your copy of the string) and you encountered the bug it means we may have another unfixed bug to find.

Let us know!

[PeterJohnson]

Thanks for the fix! No, I'm not persisting the string, so I don't think there's another bug.

A bit of background: in the actual application, I'm interfacing with an underlying API that looks something like this:

int GetNumHandles();
const char *GetString(int handle, int* length);
void SetString(int handle, const char* data, int length);

And then I have a loop over the handles like:

for (int i=0; i<GetNumHandles(); ++i) {
  GetString(...);
  if (ImGui::InputText(...)) {
    SetString(...);
  }
}

So I have to copy the buffer contents. This works if I have a static char[] that I'm copying the data into, but I'd like to support unlimited size. I think I've tested (but need to double-check) that a static std::string doesn't work in this scenario either. About the only for-sure workaround I can think of would have been to create my own array on the GUI side so I can persist a copy of every string.

[ocornut]

Ideally it would be good to be able to access that underlying data with a less restrictive API...

But another thing you could do:

1. Grab the InputText() identifier with ImGui::GetID(label).

#include "imgui_internal.h"
bool is_input_text_active = (g.ActiveId == id) && (g.InputTextState.ID == id)

Then if the bool is active, perform the copy, otherwise pass a raw char* to InputText() without any copy.

[ocornut]

I'll keep a note that I would like to solve this usage pattern better or more naturally.
Will post here when I do, in the meanwhile I'll close this issue.