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; ./a.out
// Hello World

In the preceding example, we are able to define a nested namespace without the need for separate lines.