unsigned long long(10) 无法在 clang 和 gcc 上编译

Question

The following code is rejected by both clang and gcc but accepted by msvc:

#include <iostream>

int main() 
{
    std::cout << unsigned long long(10);
}

The error is

error: expected primary-expression before 'unsigned'

godbolt

This should compile, right?

Answer 1

No, what you have shown should NOT compile. See Explicit type conversions on cppreference.com for details.

In a function-style cast, spaces are not allowed in the type name. For such types, you would need to use a C-style or C++-style cast instead, eg:

std::cout << ((unsigned long long)10);
or
std::cout << static_cast<unsigned long long>(10);

Otherwise, use a type alias instead, eg:

using ull = unsigned long long; // C++11 and later
or
typedef unsigned long long ull; // pre-C++11

std::cout << ull(10);

Note, the <cstdint> header may have a uint64_t type you can use, eg:

#include <cstdint>

std::cout << uint64_t(10);
or
std::cout << ((uint64_t)10);
or
std::cout << static_cast<uint64_t>(10);

That being said, for integer literals, you can alternatively use the ULL suffix (C++11 and later), eg:

std::cout << 10ULL;