template<typename T>

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.

Exercise Write the identity template

Write template <typename T> T identity(T x) that returns x. In main call std::cout << identity(42) << std::endl;.

Show example solution

#include <iostream>

template <typename T>
T identity(T x) {
    return x;
}

int main() {
    std::cout << identity(42) << std::endl;
    return 0;
}

Test yourself

Which keyword declares a type parameter?
```
template <___> T identity(T x);
```
- typename
- typedef
- auto
- static
Show answer

typename — typename declares a type parameter. class can be used the same way.

Complete the template declaration

template <____> T identity(T x) { return x; }

Show answer

template <typename T> T identity(T x) { return x; }

Match each piece to its role

template <typename T>

Type parameter declaration
T

Type parameter name
identity(42)

Call with int
return x

Returns the input

Next lesson: Template functions

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