I would like to give a simple tutorial on nested namespace and __has_include features in the C++17.
Nested namespace or a multi level namespace was hard in C++.
A time before C++17 the following had to be done.
namespace Framework
{
namespace ToolLayer
{
namespace Applications
{
class ToolBaseClass
{
};
}
}
}This looks huge and difficult to understand.
In C++17 it can be used as follows:
namespace Framework::ToolLayer::Applications
{
class ToolBaseClass
{
};
}__has_include Source file inclusion
The syntax for this is as follows:
__has_include ( " filename " )
__has_include ( < filename > )When the program consist of an include file with the “filename” then the statement evaluates to 1 or else it is 0. If filename which is a header file or a source file exist then it returns true.
An example shown in cppreference is here:
#if __has_include(<optional>)
# include <optional>
# define have_optional 1
#elif __has_include(<experimental/optional>)
# include <experimental/optional>
# define have_optional 1
# define experimental_optional 1
#else
# define have_optional 0
#endif
#include <iostream>
int main()
{
if (have_optional)
std::cout << "<optional> is present.\n";
int x = 42;
#if have_optional == 1
std::optional<int> i = x;
#else
int* i = &x;
#endif
std::cout << "i = " << *i << '\n';
}On a C++ 17 compiler the following output is present
<optional> is present. i = 42
Another simple example is as follows:
#if __has_include(<memory_resource>)
# include <memory_resource>
# define has_memory_resource 1
#else
# define has_memory_resource 0
#endif
#include <iostream>
int main()
{
if (has_memory_resource)
std::cout << "Memory resource is present.\n";
else
std::cout << "Memory resource is not present on versions before C++17.\n";
return 0;
}memory_resource is a new header included in C++17. If the file memory_resource is present but we use C++14 compiler then it still displays that the file is present. But if the system never had installed the C++17 then it may not have this file and in that case, it will print the message saying that it is only available for versions from C++17.
Memory resource is not present on versions before C++17.
