Partial Template Specialization

Template specialization for a single-parameter template is straightforward, but it becomes more complex with multiple parameters. This is where partial template specialization comes in. It allows you to create a specialized version of a template for specific subsets of types or values.

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// What if `T1` and `T2` are the same type?
// Or what if you want to define special behavior when `T1` is a `std::string`?

template<typename T1, typename T2>
void TemplateExample(T1 first, T2 second) 
{
    std::cout << "Generic template!" << std::endl;
}

Syntax of Partial Template Specialization

You can think of partial template specialization as a form of template overloading. Here are the rules to follow:

In partial specialization, you define a new version of a template function with some of its parameters fixed to specific types while leaving others as generic. The compiler uses this specialized version whenever it encounters matching types during template instantiation.

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#include <iostream>

// Primary template
template<typename T1, typename T2>
bool IsSameType(T1 first, T2 second) { return false; }

// Partial specialization for when both types are the same
template<typename T>
bool IsSameType(T first, T second) { return true; }

int main()
{
    std::cout << IsSameType(10, 'a') << std::endl;
    std::cout << IsSameType("Hello", "World") << std::endl;
}

That is one way to use template specialization, but there is another, more common scenario.

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#include <iostream>

// Primary template
template <typename T1, typename T2> // General case
void Template(T1 a, T2 b) { std::cout << "General" << std::endl; }

// Partial specialization for when `T1` is `bool`
template <typename T2> // Special case when the first type is `bool`
void Template(bool a, T2 b) { std::cout << "Special" << std::endl; }

int main()
{
    Template(false, 25);
    Template(100, 300);
}

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Seção 4. Capítulo 3