The C API is developed as an additional API on top of the existing C++ API. They don’t differ in many ways. This page lists common differences and dealing with them.

Namespaces and classes

Where a namespace or class name may appear, the C++ :: access operator is replaced by _, as C doesn’t have support for namespaces and classes.

For example, UImGui::Window::Platform::setWindowAlwaysOnTop() will become UImGui_Window_Platform_setWindowAlwaysOnTop() in the C API.

FVector, String and FString

The C API defines the UImGui_FVector2, UImGui_FVector and UImGui_FVector4 types. These types define geometrical vectors of floats. The C++ API actually uses a typedef to FVector2, FVector and FVector4 from the C types, under the UImGui namespace.

The FString type is not available in the C API, as templates are not a feature of C. Because of this, we’re forced to only use UImGui_String for C string representations and char* for mutable character arrays. UImGui_String is defined as String in the UImGui namespace when using the C++ API.

Classes/structs with member functions

Some classes/structs with member functions may have a custom data-only C adapter struct. This struct is then passed to the C alternatives of the same member functions to manipulate the data. Here is a C++ example:

struct doStuffWithMembers
{
    size_t num1;
    size_t num2;

    void f();
    void g();
};

This struct can then be converted to a C one like this:

struct doStuffWithMembersC
{
    size_t num1;
    size_t num2;
};

void doStuffWithMembersC_f(doStuffWithMembersC* data);
void doStuffWithMembersC_g(doStuffWithMembersC* data);

Now, using the struct in both the C++ and C way:

// C++
doStuffWithMembers cppStruct = { 0, 1 };
cppStruct.f();
cppStruct.g();

// C
doStuffWithMembersC cStruct = { 0, 1 };
doStuffWithMembersC_f(&cStruct);
doStuffWithMembersC_g(&cStruct);

This is a really common pattern used in places like the Texture and Monitor C APIs.

Dear imgui C API

The C API for dear imgui we use is based on dear_bindings. Differences in code formatting are listed here.

To use the dear imgui C API, include #include <cimgui/cimgui.h> in your code.

Dear imgui C modules API

Some dear imgui widget modules also have a C API. Refer to individual module pages for further instructions.