How am I able to use int32_t without without using std?

Question

Here is my code:

#include <iostream>

int main()
{
    int32_t i = 5;
    std::cout << "i: " << i << '\n';
}

Here is the output:

$ clang++ -std=c++11 -pedantic -Wall -Wextra foo.cpp && ./a.out 
i: 5

Here is my question:

The C++ standard appears to define int32_t in cstdint within the std namespace.

In my code, I have neither included cstdint nor do I use the std namespace. Why does the compiler not complain then?

Answer 1

The name int32_t also appears in the global scope of the C library header stdint.h. This might make it globally visible also in C++.

The section [Headers] says:

... the contents of each header cname is the same as that of the corresponding header name.h as specified in the C standard library. In the C++ standard library, however, the declarations (except for names which are defined as macros in C) are within namespace scope of the namespace std. It is unspecified whether these names (including any overloads added in [language.support] through [thread] and [depr]) are first declared within the global namespace scope and are then injected into namespace std by explicit using-declarations.

The standard also has a blanket statement:

A C++ header may include other C++ headers.

So by including <iostream> you are guaranteed to see the stream objects, but might also happen to get access to some other library features as well.

As these indirect includes are unspecified, the result varies between implementations. So the program should always include all the headers it needs, to be portable to a different compiler.