Advanced C++ Programming Cookbook

Global variables

One of the biggest limitations with header-only libraries is that, prior to C++17, there was no way to create global variables. Although global variables should be avoided whenever possible, there are situations where they are needed. To demonstrate this, let's create a simple API that outputs to stdout as follows:

#ifndef MY_LIBRARY#define MY_LIBRARY#include <iostream>#include <iomanip>namespace library_name{    void my_api(bool show_hex = false)    {        if (show_hex) {            std::cout << std::hex << "The answer is: " << 42 << '\n';        }        else {            std::cout << std::dec << "The answer is: " << 42 << '\n';        }    }}#endif

The preceding example creates an API that will output to stdout. If the API is executed with true instead of the default false