Template metaprogramming

Learn compile-time computation with template metaprogramming.

Template metaprogramming

Template metaprogramming (TMP) uses templates to do computations at compile time. A template can take a non-type parameter instead of a type: template <int N> struct Double { static const int value = N * 2; };. Here N is an int value known at compile time.

The result is a constant accessed as Double<5>::value, which equals 10. This value is baked into the executable as a ready number; no computation happens at run time. TMP is used to produce compile-time constants.

Accessing via ::value

To read the result you write the template name, the value in angle brackets and ::value. Example: Double<7>::value yields 14. The compiler computes value = 7 * 2 at compile time for Double<7>.

You can print these constants with std::cout: std::cout << Double<7>::value; prints 14. The optimizer turns this constant directly into a number.

Exercise Define the Double template

Define template <int N> struct Double with static const int value = N * 2; inside. In main write std::cout << Double<7>::value;.

Show example solution

#include <iostream>

template <int N>
struct Double {
    static const int value = N * 2;
};

int main() {
    std::cout << Double<7>::value;
    return 0;
}

Test yourself

What does template <int N> struct Double produce?
```
template <int N> struct Double { static const int value = N * 2; };
```
- A run-time variable
- A compile-time constant
- A function
- A macro
Show answer

A compile-time constant — Double<5>::value is computed as 10 at compile time.

Complete the multiplier

template <int N> struct Double { static const int value = N * ____; };

Show answer

template <int N> struct Double { static const int value = N * 2; };

Match each piece to its role

template <int N>

Non-type parameter
static const int value

Compile-time result
Double<5>::value

Constant access
struct Double

Template struct

Next lesson: Recursive templates

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