Multiple type parameters

Learn multiple types with template<typename T, typename U>.

Multiple type parameters

A template can take multiple type parameters. template <typename T, typename U> struct Pair { T first; U second; }; declares two type parameters, separated by commas.

Example: Pair<int, double> p; makes first int and second double. p.first = 3; p.second = 3.14; sets the members. The type order matches the declaration: the first type maps to T, the second to U. STL's std::pair and std::map use this pattern.

Using Pair

Instantiate Pair: Pair<int, double> p;. Members are accessed directly: p.first = 3; p.second = 3.14;. The type order must match the declaration.

Wrong order: Pair<double, int> p; p.first = 3.14; p.second = 3; — here first is double and second is int. The types swap. You read which type maps to which member from the declaration. std::pair is often used as a key-value pair in STL algorithms.

Exercise Write the Pair template

Write template <typename T, typename U> struct Pair. Inside, have T first; and U second;. In main write Pair<int, double> p;, p.first = 3;, std::cout << p.first << std::endl;.

Show example solution

#include <iostream>

template <typename T, typename U>
struct Pair {
    T first;
    U second;
};

int main() {
    Pair<int, double> p;
    p.first = 3;
    p.second = 3.14;
    std::cout << p.first << std::endl;
    return 0;
}

Test yourself

How many type parameters does Pair<int, double> take?
```
template <typename T, typename U> struct Pair;
```
- Zero
- One
- Three
- Two
Show answer

Two — Pair<int, double> takes two type parameters: T and U.
Complete the Pair declaration
```
Pair<int, ____> p;
```
Show answer
```
Pair<int, double> p;
```
Match each piece to its role

typename T, typename U

Two type parameters
T first

First type member
U second

Second type member
Pair<int, double>

Instantiation

Next lesson: Template specialization

We Are Enhancing Your Experience

Cookie Preferences

Strictly Necessary Cookies

Analytical Cookies