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The All-in-One Guide to C++20

Four Ways to Use Concepts

Explore four distinct ways to use concepts in C++20, including requires clauses and constrained template parameters, to write safer and more generic template functions. Understand how concepts improve type constraints and template behavior for integral types.

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Before discussing the four ways to use a concept, look at the code below. Here, I apply the predefined concept std::integral in all four ways.

C++
#include <concepts> 
#include <iostream>
// Requires clause
template<typename T> 
requires std::integral<T> 
auto gcd(T a,T b) {
    if( b == 0 ) return a;
    else return gcd(b, a % b); 
}
// Trailing requires clause
template<typename T> 
auto gcd1(T a,T b) requires std::integral<T> {
    if( b == 0 ) return a;
    else return gcd1(b, a % b); 
}
// Constrained template parameter
template<std::integral T> 
auto gcd2(T a,T b) {
    if( b == 0 ) return a;
    else return gcd2(b, a % b); 
}
// Abbreviated function template
auto gcd3(std::integral auto a, std::integral auto b) { 
    if( b == 0 ) return a;
    else return gcd3(b, a % b);
} 
int main() {
    std::cout << '\n';
    std::cout << "gcd(100, 10)= " << gcd(100, 10) << '\n';
    std::cout << "gcd1(100, 10)= " << gcd1(100, 10) << '\n';
    std::cout << "gcd2(100, 10)= " << gcd2(100, 10) << '\n';
    std::cout << "gcd3(100, 10)= " << gcd3(100, 10) << '\n';
    std::cout << '\n';
}

Thanks to the header <concepts> at line 1, I can use the concept std::integral. The concept is fulfilled if T is the type integral. The function name gcd stands for the greatest common divisor algorithm based on the Euclidean algorithm.

Here are the four ways to use concepts:

  • Requires clause (at line 5
...