template_alias

Concept: template alias

Template aliases in C++ provide a way to create an alias (or shorthand) for a templated type, making the code more readable and easier to manage, especially when dealing with complex types. This feature was introduced in C++11 and allows you to define a type alias using the using keyword. Template aliases are particularly useful when you have long or complex type names that you want to simplify for repeated use.

Syntax of Template Aliases

The basic syntax for creating a template alias is:

template<typename T>
using AliasName = ExistingTemplate<T>;

• template<typename T>: Defines the template parameter(s).
• using AliasName = ExistingTemplate<T>;: Creates an alias AliasName for the template ExistingTemplate<T>.

Example 1: Basic Template Alias

Let’s say you have a complex template like std::map<T, std::vector<T>>. You can create an alias to simplify the usage of this type:

#include <iostream>
#include <map>
#include <vector>

template<typename T>
using MapOfVectors = std::map<T, std::vector<T>>;

int main() {
    MapOfVectors<int> myMap;

    myMap[1].push_back(10);
    myMap[1].push_back(20);

    for (const auto& item : myMap[1]) {
        std::cout << item << " ";
    }

    return 0;
}

Explanation:

• template<typename T> using MapOfVectors = std::map<T, std::vector<T>>; creates a template alias MapOfVectors for the type `std::map>.
• Now, instead of writing std::map>, you can simply useMapOfVectors`.

Example 2: Alias for a Function Pointer Type

Suppose you have a function pointer type that you use frequently. Instead of repeatedly writing out the full type, you can create a template alias for it.

#include <iostream>

// A function pointer type: int(*)(T, T)
template<typename T>
using Comparator = int(*)(T, T);

int compareInts(int a, int b) {
    return a - b;
}

int main() {
    Comparator<int> comp = &compareInts;

    std::cout << "Comparison result: " << comp(10, 20) << std::endl;

    return 0;
}

Explanation:

• template<typename T> using Comparator = int(*)(T, T); creates an alias Comparator for a function pointer type that takes two arguments of type T and returns an int.
• You can now use Comparator<int> as a shorthand for int(*)(int, int).

Example 3: Simplifying Nested Templates

Consider a scenario where you are dealing with nested template types like std::vector<std::pair<T1, T2>>. A template alias can simplify this:

#include <iostream>
#include <vector>
#include <utility>

template<typename T1, typename T2>
using PairVector = std::vector<std::pair<T1, T2>>;

int main() {
    PairVector<int, std::string> vec;

    vec.push_back({1, "One"});
    vec.push_back({2, "Two"});

    for (const auto& item : vec) {
        std::cout << item.first << " : " << item.second << std::endl;
    }

    return 0;
}

Explanation:

• template<typename T1, typename T2> using PairVector = std::vector<std::pair<T1, T2>>; creates a template alias PairVector for the nested template type `std::vector>.
• You can now use PairVector<int, std::string> instead of writing std::vector<std::pair<int, std::string>>.

Example 4: Alias for Template Classes with Default Parameters

Template aliases can also be used with templates that have default parameters. This can be particularly useful when you want to provide an alias that includes a commonly used default parameter.

#include <iostream>
#include <map>

template<typename T, typename Compare = std::less<T>>
using SortedMap = std::map<T, int, Compare>;

int main() {
    SortedMap<int> myMap;  // std::map<int, int, std::less<int>> by default

    myMap[3] = 30;
    myMap[1] = 10;
    myMap[2] = 20;

    for (const auto& item : myMap) {
        std::cout << item.first << " : " << item.second << std::endl;
    }

    return 0;
}

Explanation:

• template<typename T, typename Compare = std::less<T>> using SortedMap = std::map<T, int, Compare>; creates an alias SortedMap for a std::map with std::less<T> as the default comparator.
• This alias makes it easier to work with a map sorted by keys, where the default comparator is often std::less.

Summary

• Template Aliases: Allow you to create simple, readable aliases for complex template types using the using keyword.
• Improved Readability: Aliases make code more readable and reduce redundancy, especially when dealing with long or nested template types.
• Function Pointers and Nested Templates: Template aliases are particularly useful for simplifying function pointer types and deeply nested template structures.
• Default Template Parameters: Template aliases can also include default template parameters, making it easier to provide commonly used types with default settings.

Template aliases are a powerful feature in modern C++ that can significantly improve code clarity and maintainability, especially when working with complex template types.