Search⌘ K
AI Features
Home
Courses
C++ Concepts: Improve Type Safety with C++ 20

Negations Using Concepts

Understand how negations function within C++ Concepts, especially how the opposite of a true concept expression may not be false but ill-formed. Learn to properly use parentheses in negations to write valid and clear template constraints, enhancing your ability to create precise and safe generic code.

We'll cover the following...

!a is not the opposite of a

When we talk about concepts, the opposite of a true expression is not an expression that is evaluated to false. Let’s suppose we have a function foo() that takes two parameters, T bar and U baz. We have some constraints on them. One of them must have a nested type Blah that is unsigned.

C++
#include <concepts>
template <typename T, typename U>
requires std::unsigned_integral<typename T::Blah> ||
std::unsigned_integral<typename U::Blah>
void foo(T bar, U baz) {
    // ...
}
class MyType {
public:
    using Blah = unsigned int;
    // ...
};
int main() {
    MyType mt;
    foo(mt, 5);
    foo(5, mt);
    // error: no operand of the disjunction is satisfied
    // foo(5, 3);
}

When we call foo() on line 18 with an instance of MyType in the ...