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C++ function templates

Reuse one type-checked algorithm for different types.

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Before you start

Functions, comparisons, and return values.

Reuse one type-checked algorithm for different types.
Predict the sample output and explain the state changes.
Complete the practice task and check the boundary cases.

The concept

A function template describes a family of functions. The compiler deduces T from matching arguments and instantiates the needed versions.

This maximum implementation requires that T supports comparison with > and copying its selected value. Mixed argument types need an explicit design decision rather than an accidental conversion.

Code example

C++

#include <iostream>
template <typename T>
T maximum(T left, T right) {
    return left > right ? left : right;
}
int main() {
    std::cout << maximum(3, 7) << '\n';
    std::cout << maximum(2.5, 1.5) << '\n';
}

Output

7
2.5

Example details

Follow the code

Open the walkthrough to inspect the example one step at a time.

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Guided walkthroughReady

1#include <iostream>

2template <typename T>

3T maximum(T left, T right) {

4 return left > right ? left : right;

5}

6int main() {

7 std::cout << maximum(3, 7) << '\n';

8 std::cout << maximum(2.5, 1.5) << '\n';

9}

Follow the example

Start to inspect the source, values, and output. These prepared steps explain the published sample; grouped loop steps describe multiple iterations.

View the complete dry-run table

Prepared trace of the published program. Some rows group repeated iterations.
What happens	State after step	Output so far
Deduce T as int.	`left = 3, right = 7`	—
3 > 7 is false, so choose right.	`return = 7`	—
Print the int result.	`return = 7`	7
Deduce T as double; 2.5 is greater.	`return = 2.5`	7 2.5

Behind the scenes

How C++ processes this code

The C++ compiler produces a native executable. Execution enters main, where objects and function calls perform the work.

Cost and common mistakes

One comparison plus copying; O(1) for the primitive types shown.

maximum(3, 2.5) cannot deduce one T in this declaration.
Templates do not guarantee that every possible type supports the operations in the body.

Try it yourself

Predict maximum(4, 4).

Reveal answer and reasoning

It returns 4; the comparison is false and the right value is selected.