Hands-On System Programming with C++

Namespaces

A welcome change to C++17 is the addition of nested namespaces. Prior to C++17, nested namespaces had to be defined on different lines, as follows:

#include <iostream>namespace X {namespace Y{namespace Z {    auto msg = "Hello World\n";}}}int main(void){    std::cout << X::Y::Z::msg;}// > g++ scratchpad.cpp; ./a.out// Hello World

In the preceding example, we define a message that is output to stdout in a nested namespace. The problem with this syntax is obvious—it takes up a lot of space. In C++17, this limitation was removed by giving us the ability to declare nested namespaces on the same line, as follows:

#include <iostream>namespace X::Y::Z {    auto msg = "Hello World\n";}int main(void){    std::cout << X::Y::Z::msg;}// > g++ scratchpad.cpp; ...