Templates

Basics

#include <iostream>
#include <string>
 
using namespace std;
 
struct Job {
    int salary;
    string employee;
};
 
template <typename T>
void Swap(T& a, T& b) {
    T tmp = a;
    a = b;
    b = tmp;
}
 
template void Swap<string>(string&, string&); // explicit instantiation
 
template <> void Swap<Job>(Job& j1, Job& j2) { // explicit specialization
    int tmp = j1.salary;
    j1.salary = j2.salary;
    j2.salary = tmp;
} 
 
int main() {
    
    // implicit instantiation for int
    int a = 10, b = 4;
    cout << a << " " << b << endl;
    Swap(a, b);
    cout << a << " " << b << endl;
 
    // explicit instantiation for string
    string sa = "jimmy";
    string sb = "lin";
    cout << sa << " " << sb << endl;
    Swap(sa, sb);
    cout << sa << " " << sb << endl;
 
    // use explicit specialization for struct Job
    struct Job j1 = {.salary = 100, .employee = "james"};
    struct Job j2 = {.salary = 200, .employee = "john"};
    cout << j1.employee << " " << j1.salary << endl;
    cout << j2.employee << " " << j2.salary << endl;
    Swap(j1, j2);  // use explicit specialization
    cout << j1.employee << " " << j1.salary << endl;
    cout << j2.employee << " " << j2.salary << endl;
    return 0;
}

#include <iostream>
using namespace std;
 
template <typename T>
T Add(T a, T b) {
    return a + b;
}
 
int main() {
    
    // implicit instantiation for int
    int a = 10, b = 4;
    cout << a << " " << b << endl;
    auto c = Add(a, b);
    cout << c << endl;
 
    // explicit instantiation of double
    double d = 4.56;
    auto e = Add<double>(a, d);
    cout << e << endl;
    return 0;
}

Type Deduction

• template type deduction

Variadic Template

• 參數展開的方式
  • 遞迴
    • 要寫 base case
    • C++17: using constexpr
  • Fold expressions (since C++17)

Recursion example:

#include <iostream>
 
template<typename T0, typename ...T>
auto sum(T0 t0, T... t) {
    if constexpr (sizeof...(t) > 0)
        return t0 + sum(t...);
    return t0;
}
 
int main() {
    std::cout << sum(1, 2, 3, 2, 5) << std::endl;
}

Fold Expression example:

#include <iostream>
 
template<typename ...T>
auto sum(T ...t) {
    return (t + ...);
}
 
template<typename ...T>
auto average(T ...t) {
    return (t + ...) / sizeof...(t);
}
 
int main() {
    std::cout << sum(1, 2, 3, 2.3, 5) << std::endl;
    std::cout << average(1, 2, 3, 2.3, 5) << std::endl;
}

auto returned type (C++ 14)

#include <concepts>
 
template<typename T1, typename T2>
auto myMax(T1 a, T2 b) {
    return a < b ? a : b;
}
 
int main() {
    auto m = myMax(1, 2);
    auto m2 = myMax(1, 2.9);
}

auto as function parameters(c++20) & Concepts

Since C++20, we can use concepts to specify what we want with the template we defined.

auto 相較於傳統的 template，給了我們更多自由，但同時也會造成 ambiguity，所以需要配合 Concepts 去限制它。

下面的例子中，若不使用 HasPushBack ，就會造成 ambiguity，compiler 會不知道該 call 哪一個 add function。

#include <concepts>
#include <set>
#include <vector>
#include <concepts> 
 
template<typename Container>
concept HasPushBack = requires (Container c, Container::value_type v) {
    c.push_back(v);
};
 
void add(HasPushBack auto& container, const auto& val) {
    container.push_back(val);
}
 
void add(auto& container, const auto& val) {
    container.insert(val);
}
 
int main() {
    std::vector<int> container1;
    std::set<int> container2;
 
    add(container1, 22);
    add(container2, 22);
}

配合 constexpr（compile-time if）以上的 code 可以進一步改寫成：

#include <concepts>
#include <set>
#include <vector>
#include <concepts> 
 
void add(auto& container, const auto& val) {
    if constexpr (requires {container.push_back(val);})
        container.push_back(val);
    else
        container.insert(val);
}
 
int main() {
    std::vector<int> container1;
    std::set<int> container2;
 
    add(container1, 22);
    add(container2, 22);
}

Reference

• Back to Basics: Templates in C++ - Nicolai Josuttis - CppCon 2022