To create a header-only library, simply ensure that all of your code is implemented in header files, as follows:

#ifndef MY_LIBRARY
#define MY_LIBRARY

namespace library_name
{
    int my_api() { return 42; }
}

#endif

The preceding example implements a simple library with a single function. The entire implementation of this library can be implemented in a single header file and included in our code as follows:

#include "my_library.h"
#include <iostream>

int main(void)
{
    using namespace library_name;

    std::cout << "The answer is: " << my_api() << '
';
    return 0;
}

Although creating header-only libraries seems simple enough, there are some issues that arise when attempting to create a header-only library that should be taken into account.