Using std::string in ImGui::InputText(...) - c++

Question

The call to ImGui::InputText() takes a char array which I need to initialise from a std::string and then transfer the contents back to the std::string. In it's simplest form:
char buf[255]{};
std::string s{"foo"};
void fn() {
strncpy( buf, s.c_str(), sizeof(buf)-1 );
ImGui::InputText( "Text", buf, sizeof(buf) );
s=buf;
}
However, it appears wasteful to have two buffers (buf and the buffer allocated within std::string) both doing much the same thing. Can I avoid the buf buffer and the copying to and from it by using just the std::string and a simple wrapper "X".
I don't care about efficiency, I just want the simplest code at the call site.
This code does work but is it safe and is there a better way?
class X {
public:
X(std::string& s) : s_{s} { s.resize(len_); }
~X() { s_.resize(strlen(s_.c_str())); }
operator char*(){ return s_.data(); }
static constexpr auto len() { return len_-1; }
private:
std::string& s_;
static constexpr auto len_=255;
};
std::string s{"foo"};
void fn() {
ImGui::InputText( "Text", X(s), X::len() );
}

Answer 1

If you want to use InputText() with std::string or any custom dynamic string type, see misc/cpp/imgui_stdlib.h and comments in imgui_demo.cpp.
misc/cpp/imgui_stdlib.h
namespace ImGui
{
// ImGui::InputText() with std::string
// Because text input needs dynamic resizing, we need to setup a callback to grow the capacity
IMGUI_API bool InputText(const char* label, std::string* str, ImGuiInputTextFlags flags = 0, ImGuiInputTextCallback callback = NULL, void* user_data = NULL);
IMGUI_API bool InputTextMultiline(const char* label, std::string* str, const ImVec2& size = ImVec2(0, 0), ImGuiInputTextFlags flags = 0, ImGuiInputTextCallback callback = NULL, void* user_data = NULL);
IMGUI_API bool InputTextWithHint(const char* label, const char* hint, std::string* str, ImGuiInputTextFlags flags = 0, ImGuiInputTextCallback callback = NULL, void* user_data = NULL);
}
Your first code
std::string s{"foo"};
void fn() {
ImGui::InputText( "Text", &s );
}
Reading manuals works wonders.