Template instantiation
- C++ course
- Expert
- Lesson 39/47
- 5 min
Learn the template instantiation mechanism.
Template instantiation
Template instantiation is the compiler generating concrete code from a template. The template itself is not code; it is a recipe. For each type used, concrete code is generated.
There are two kinds: implicit — the compiler generates automatically when the template is used; explicit — the programmer forces generation with template class Foo<int>;. Explicit instantiation is used to reduce compile time and solve translation-unit boundaries.
Implicit instantiation
The template template <typename T> T identity(T x) { return x; } generates no code when declared. When identity(7) is first called, the compiler generates the identity<int> version.
If called with a second type (identity(3.14)), the compiler generates identity<double>. So a separate concrete function is generated for each distinct type. This is why templates must be fully in a header; otherwise the compiler cannot see uses in other translation units.
Practice Write the identity template
Write template <typename T> T identity(T x) with body return x;. In main write std::cout << identity(7);.
Starter code
#include <iostream>
// Define the identity template function here
int main() {
// Call identity and print it
return 0;
}Sample solution
#include <iostream>
template <typename T>
T identity(T x) {
return x;
}
int main() {
std::cout << identity(7);
return 0;
}Expected output
7
Questions and answers
Review the lesson questions together with their answers. To test yourself, solve the lesson interactively.
1 Match each concept to its role
| Term | Meaning |
|---|---|
implicit instantiation |
Automatic at use site |
explicit instantiation |
template class Foo<int>; |
type deduction |
auto x = ... |
specialization |
Special behavior |
2 Complete the explicit instantiation
template ____ Vector<int>;template class Vector<int>;Explanation: template class Vector<int>; declares explicit instantiation.
3 When does implicit template instantiation happen?
template <typename T> T identity(T x) { return x; }
auto a = identity(7);- When the compiler sees the template
- During linking
- When the template is used (called) Correct answer
- At run time
Explanation: In implicit instantiation, code is generated when the template is first used.