Concept overloading
- C++ course
- Advanced
- Lesson 65/83
- 5 min
Learn function overloading with concepts.
What is concept overloading?
Concept overloading is defining the same template function name several times with different concept constraints. At a call site the compiler picks the best overload based on which concept the type satisfies.
Example: a print function can be written separately for integral and floating_point types. template<std::integral T> void print(T v) and template<std::floating_point T> void print(T v). print(3) calls the integral version, print(2.5) calls the floating_point version. Type-based behaviour is then dispatched at compile time.
More specialised concept
In concept overloading, a more specialised concept is preferred over a more general one. std::integral is more general; std::signed_integral is more specialised.
Example: if both std::integral and std::signed_integral versions exist, print(-3) goes to the signed version. Because int is both integral and signed_integral; signed_integral is more specific. The compiler makes this choice with partial ordering. The correct version is then picked without ambiguity.
Practice Write integral and floating_point print
Write two print overloads: first template<std::integral T> void print(T v) with body std::cout << "i:" << v;; second template<std::floating_point T> void print(T v) with body std::cout << "f:" << v;. In main call print(3).
Starter code
#include <iostream>
#include <concepts>
// Write the integral print overload
// Write the floating_point print overload
int main() {
// Make the first call
return 0;
}Sample solution
#include <iostream>
#include <concepts>
template<std::integral T>
void print(T v) {
std::cout << "i:" << v;
}
template<std::floating_point T>
void print(T v) {
std::cout << "f:" << v;
}
int main() {
print(3);
return 0;
}Expected output
i:3
Questions and answers
Review the lesson questions together with their answers. To test yourself, solve the lesson interactively.
1 How is the overload chosen?
- By which concept the type satisfies Correct answer
- It is chosen randomly
- The type is queried at runtime
- By the order of function definitions
Explanation: The compiler checks which concept the call type satisfies and picks the most specialised version.
2 Match the call to the type
| Term | Meaning |
|---|---|
print(3) |
int call |
print(2.5) |
double call |
print('a') |
char call |
print(3.14f) |
float call |
3 Floating-point constraint
template<____ T> void print(T v) { std::cout << v; }template<std::floating_point T> void print(T v) { std::cout << v; }Explanation: std::floating_point accepts only floating-point types.