Blah.h

namespace api
{
    namespace blah
    {
        // Opaque handle to the type this namespace exposes functions for
        struct Blah;
        
        // Lifetime management (malloc/free with construct/destruct)
        Blah* New(u32 param);
        void Delete(Blah* blah);
        
        // 'Blah' API
        void DoSomething(Blah* blah, u32 param);
        u32 GetSomething(const Blah* blah);
    }
    
    // Bring the opaque handle into the API namespace
    using blah::Blah;
}

Main.cpp

#include "Blah.h"

int main()
{
    api::Blah* blah = api::blah::New(2);
    
    // All remaining work can be done with Koenig Lookup (ADR)
    DoSomething(blah, 1);
    u32 x = GetSomething(blah);
    Delete(blah);
    
    return 0;
}