Is it possible to create a vector of concept ? std::vector<Concept auto>

Question

I'm trying to study the concept functionality and syntax, but I failed to come up with a working example of std::vector, any idea of what is going wrong? https://gcc.godbolt.org/z/FHaQ-3

#include <vector>
#include <string>

template<typename T>
concept Compare = requires(T a, T b) {
    { a <=> b } -> std::same_as<std::partial_ordering>;
};

struct Cat {
    int age;
    std::string name;
    auto operator<=>(const Cat&) const = default;
};

int main(int argc, char** argv) {
    /* Single type: Fail
    std::vector<Compare auto> vec{
        Cat{4, "Faisca"},
        Cat{4, "Neka"}
    };

    */
    /* Heterogenous: Fail
    std::vector<Compare auto> vec{
        Cat{4, "Faisca"}, 1, std::string{"Deu Ruim"}
    };
    */
    return 0;
}

Answer 1

You can't have a std::vector of heterogenous types.

However, you can have a std::vector of type T that satisfies a concept. Simply provide a alias template that is constrained:

template<typename T>
 requires Compare<T>
  using vec = std::vector<T>;

or with terse syntax:

template<Compare T>
 using vec = std::vector<T>;

Now, you can do:

vec<Cat> v;  

but not

vec<int> v;

Note that your Cat class as currently written, doesn't actually satisfy the concept Compare . If you change the return type of operator<=> , like this:

std::partial_ordering operator<=>(const Cat&) const = default;

you can create a vector of Cat s:

vec<Cat> a{
   Cat{4, "Faisca"},
   Cat{4, "Neka"}
};

Answer 2

You cannot use placeholder types ( auto and decltype(auto) , with or without constraints) within template argument lists like that. You must either use a concrete type or rely on class template argument deduction from C++17 (by not specifying template arguments at all).

A vector<T> is a sequence container, and sequence containers contain objects of a given type. A single given type.