Cross platrform way to identify current opengl rendering context? - opengl

Question

I have discovered that some OpenGL objects, such as shaders, are not shared between rendering contexts and will raise a GL_INVALID_VALUE error if used in the wrong context.
So my library has something like this (assume C++ wrappers around the OpenGL IDs):
const char * const VERTEX_SHADER_LIB = "...";
const char * const FRAGMENT_SHADER_LIB = "...";
ShaderProgram shader (FragmentShader plugin)
{
ShaderProgram p;
p .attach (VerexShader (VERTEX_SHADER_LIB));
p .attach (FragmentShader (FRAGMENT_SHADER_LIB));
p .attach (plugin);
p .link ();
return p;
}
In the interests of efficiency, I rewrite it along these lines:
VertexShader vertex_shader_lib ()
{
static VertexShader SHADER (VERTEX_SHADER_LIB);
return SHADER;
}
FragmentShader fragment_shader_lib ()
{
static FragmentShader SHADER (FRAGMENT_SHADER_LIB);
return SHADER;
}
ShaderProgram shader (FragmentShader plugin)
{
ShaderProgram p;
p .attach (vertex_shader_lib ());
p .attach (fragment_shader_lib ());
p .attach (plugin);
p .link ();
return p;
}
but this more efficient version only works if used for a single rendering context.
I would therefore need to rewrite it like this:
VertexShader vertex_shader_lib ()
{
static Map <RenderContextID, VertexShader> SHADERS;
SHADERS .insert_if_key_does_not_exist (
current_render_context (),
VertexShader (VERTEX_SHADER_LIB));
return SHADERS [current_render_context ()];
}
however, I can't find a cross-platform way to describe current_render_context(). Is there one? I don't necessarily need a cross-platform handle for the rendering context object itself, but any piece of identifying data which is unchanging and unique to each context will do.
What can I use?

Answer 1

Use a platform-appropriate *GetCurrentContext() function:
// Windows
HGLRC wglGetCurrentContext(void)
// X11
GLXContext glXGetCurrentContext(void)
// OSX
CGLContextObj CGLGetCurrentContext(void)
// EGL
EGLContext eglGetCurrentContext(void)
It'll take a little bit of #ifdefing but nothing too onerous.
If you use a framework like SDL2 you can avoid even that.