How can I (static_)assert that string is not a type?

Question

I am working in an environment with many, many header files, into some of which sometimes a using namespace std; sneaks in. I want to catch (in CI) when that does. It is easy to check that something does exist/compile, but the contrary is surprisingly difficult.

My idea was this:

#include "header1.h"
#include "header2.h"
// ...

static_assert(!type_is_defined(string))

How can I write type_is_defined such that it compiles when string is not defined?

Answer 1

This may not be the most elegant way, but it works:

#include <string>
#include <type_traits>

//... many headers here, some of which may contain 'using namespace std;'
//using namespace std; //uncommenting it causes compiler error: "reference to 'string' is ambiguous" 

class string;
static_assert(!std::same_as<string, std::string>);

The static_assert requires C++20 because of same_as concept. For C++17, you need to replace it with is_same_v. If you use an earlier C++ version, you can use the following (which works for C++11):

static_assert(!std::is_same<string, std::string>::value, "!");