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C++ Concepts: Improve Type Safety with C++ 20

Concepts at Compile-time

Explore how C++20 Concepts are compiled in functions using four different syntaxes. Understand their equivalences, the impact on template specialization, and how these approaches produce identical assembly code, setting a foundation for more complex usages in generic programming.

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How concepts are compiled

Because we have four different ways to use concepts with functions, we might ask ourselves how they are each compiled. Can there be such a subtle reason behind allowing all these different syntaxes?

Nothing is simpler than verifying that! Let’s start first with the code and then see what goes on behind the scenes.

C++
#include <concepts>
#include <iostream>
template <typename T>
concept Number = std::integral<T> || std::floating_point<T>;
template <typename T>
//the <requires> clause
requires Number<T>
auto addRequiresClause(T a, T b) {
  return a+b;
}
template <typename T>
//trailing <requires> clause
auto addTrailingRequiresClause(T a, T b) requires Number<T> {
  return a+b;
}
template <Number T>
//constrained template parameter
auto addConstrainedTemplate(T a, T b) {
  return a+b;
}
//abbreviated function template
auto addAbbreviatedFunctionTemplate(Number auto a, Number auto b) {
  return a+b;
}
int main() {
    std::cout << "addRequiresClause(1, 2): " << addRequiresClause(1, 2) << '\n';
    std::cout << "addTrailingRequiresClause(1, 2): " << addTrailingRequiresClause(1, 2) << '\n';
    std::cout << "addConstrainedTemplate(1, 2): " << addConstrainedTemplate(1, 2) << '\n';
    std::cout << "addAbbreviatedFunctionTemplate(1, 2): " << addAbbreviatedFunctionTemplate(1, 2) << '\n';
    std::cout << "addAbbreviatedFunctionTemplate(1, 2): " << addAbbreviatedFunctionTemplate(1, 2.14) << '\n';
}

Explanation

The first and third methods become the same.

template<>
int addRequiresClause<int>(int a, int b)
{
    return a +
...