Experimental attempt to create an ImGui::InputTextMultiline(...) for std::strings

imgui_input_text_multiline_for_std_vector.h.cpp

// EXPERIMENTAL (ALMOST UNTESTED) InputTextMultiline(...) for std::strings.
// USE IT AT YOUR OWN RISK!

// Example usage:
/*
    static std::string text = "Dear ImGui lacks InputTextMultiline(...) for std::string.";
    static bool isTextBoxActive = false;  // A better implementation should remove this
    ImGui::InputTextMultiline("###ADummyLabelForMe",text,isTextBoxActive);
*/


// .h file
#include <imgui.h>
#include <string>
namespace ImGui {
IMGUI_API bool InputTextMultiline(const char* label,std::string& text,bool& staticItemActiveInOut, const ImVec2& size = ImVec2(0,0), ImGuiInputTextFlags flags = 0, ImGuiTextEditCallback callback = NULL, void* user_data = NULL);
}

// .cpp file (needs imgui_internal.h)
namespace ImGui {

inline static void StdStringNonStdResize(std::string& s,int size) {
    IM_ASSERT(size>=0);
    const int oldLength = s.length();
    if (size<oldLength) s = s.substr(0,size);
    else if (size>oldLength) for (int i=0,icnt=size-oldLength;i<icnt;i++) s+='\0';
}

// Ideally we should remove "staticItemActiveInOut" and find a way to query the active state BEFORE calling the default InputTextMultiline(...) version
// Also I'm not sure this works well with Undo/Redo
// And I've only tested it in a single instance
bool InputTextMultiline(const char* label,std::string& text,bool& staticItemActiveInOut, const ImVec2& size, ImGuiInputTextFlags flags, ImGuiTextEditCallback callback, void* user_data)    {
    bool rv = false;
    const bool is_editable = (flags & ImGuiInputTextFlags_ReadOnly) == 0;
    if (staticItemActiveInOutForID && is_editable) {
        const ImGuiIO& io = ImGui::GetIO();
        ImGuiTextEditState &edit_state = ImGui::GetCurrentContext()->InputTextState;    // Hope this always points to the active InputText...
        const char* clipText = (io.KeyMap[ImGuiKey_V] && io.KeyCtrl) ? ImGui::GetClipboardText() : "";
        const size_t clipSize = strlen(clipText);
        size_t sizeToAdd = clipSize+((io.InputCharacters[0] && (!(io.KeyCtrl && !io.KeyAlt)))?(5*IM_ARRAYSIZE(io.InputCharacters)):0);
        if (IsKeyPressedMap(ImGuiKey_Enter) || IsKeyPressedMap(ImGuiKey_Tab)) sizeToAdd+=5;
        size_t cnt = 0;
        if (sizeToAdd>0)    {
            // Note that this happens only on a few frames
            sizeToAdd+=1;   // Trailing '\0'
            const size_t oldTextLen = (size_t)text.length();
            const size_t newTextLen = oldTextLen+sizeToAdd;
            StdStringNonStdResize(text,newTextLen);  // See its code: it sets text[i]='\0' too
            edit_state.Text.resize(text.length()+1);
            edit_state.InitialText.resize(text.length()+1);

            rv = ImGui::InputTextMultiline(label,&text[0],newTextLen,flags,callback,user_data);

            // Remove all trailing '\0's and resize all strings back to save memory (a bit slow)
            for (int i=(int)text.length()-1;i>=0;i--) {
                if (text[i]!='\0') break;
                ++cnt;
            }
            StdStringNonStdResize(text,text.length()-cnt);

            staticItemActiveInOutForID = ImGui::IsItemActive();
            if  (staticItemActiveInOutForID)    // Otherwise maybe edit_state points to something else
            {
                //Allows the textbox to expand while active.
                edit_state.Text.resize(text.length()+1);
                edit_state.InitialText.resize(text.length()+1);
            }
        }
        else {
            rv = ImGui::InputTextMultiline(label,&text[0],text.length(),flags,callback,user_data);
            staticItemActiveInOutForID = ImGui::IsItemActive();
        }

        // Dbg only: (to remove)
        if (ImGui::IsItemHovered()) ImGui::SetTooltip("text.length():%d (%s) cnt:%d sizeToAdd:%d\n",text.length(),staticItemActiveInOutForID?"active":"inactive",cnt,(int)sizeToAdd);

    }
    else {
        rv = ImGui::InputTextMultiline(label,&text[0],text.length(),flags,callback,user_data);
        staticItemActiveInOutForID = ImGui::IsItemActive();
    }
    return rv;
}

} // namespace ImGui

Intended for ImGui version 1.52.

Please see: ocornut/imgui#1008 and ocornut/imgui#1443