template<typename T>
- C++ course
- Intermediate
- Lesson 47/116
- 5 min
Learn to write generic functions with template<typename T>.
What is a template?
A template lets you write type-independent code. The template <typename T> declaration introduces a type parameter named T. The compiler generates a separate version for each type used.
Example: template <typename T> T identity(T x) { return x; } works for int, double and std::string. When you call identity(42), the compiler generates a version with T=int. This is called instantiation.
Type deduction
You don't have to specify the type when calling a template function. In identity(42) the compiler deduces T=int (type deduction). This keeps code concise.
You can also specify the type explicitly: identity<double>(3.14). This is needed when deduction would be ambiguous. For example, identity(3.14) deduces T=double. Templates spare you from writing the same function for many types.
Practice Write the identity template
Write template <typename T> T identity(T x) that returns x. In main call std::cout << identity(42) << std::endl;.
Starter code
#include <iostream>
// write the identity template function: T identity(T x)
int main() {
// call identity and print the result
return 0;
}Sample solution
#include <iostream>
template <typename T>
T identity(T x) {
return x;
}
int main() {
std::cout << identity(42) << std::endl;
return 0;
}Expected output
42
Questions and answers
Review the lesson questions together with their answers. To test yourself, solve the lesson interactively.
1 Which keyword declares a type parameter?
template <___> T identity(T x);- typename Correct answer
- typedef
- auto
- static
Explanation: typename declares a type parameter. class can be used the same way.
2 Complete the template declaration
template <____> T identity(T x) { return x; }template <typename T> T identity(T x) { return x; }Explanation: template <typename T> declares T as a type parameter.
3 Match each piece to its role
| Term | Meaning |
|---|---|
template <typename T> |
Type parameter declaration |
T |
Type parameter name |
identity(42) |
Call with int |
return x |
Returns the input |