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What we just learned about was implicit template type deduction. In some cases, we cannot rely on implicit type deduction. Consider the following example class:

template <typename T>
struct sum {
    T value;

    template <typename ... Ts>
    sum(Ts&& ... values) : value{(values + ...)} {}
};

This struct, sum, accepts an arbitrary number of parameters and adds them together using a fold expression (have a look at the fold expression recipe a little later in this chapter to get more details on fold expressions). The resulting sum is saved in the member variable value. Now the question is, what type is T? If we don't want to specify it explicitly, it surely needs to depend on the types of the values provided in the constructor. If we provide string instances, it needs to be std::string. If we provide integers, it needs to be int. If we provide integers, floats, and doubles, the compiler needs to figure out which type fits all the values without information loss. In order to achieve that, we provide an explicit deduction guide:

template <typename ... Ts>
sum(Ts&& ... ts) -> sum<std::common_type_t<Ts...>>;

This deduction guide tells the compiler to use the std::common_type_t trait, which is able to find out which common type fits all the values. Let's see how to use it:

sum s          {1u, 2.0, 3, 4.0f};
sum string_sum {std::string{"abc"}, "def"};

std::cout << s.value          << '\n'
          << string_sum.value << '\n';

In the first line we instantiate a sum object with constructor arguments of type unsigned, double, int, and float. The std::common_type_t returns double as the common type, so we get a sum<double> instance. In the second line, we provide an std::string instance and a C-style string. Following our deduction guide, the compiler constructs an instance of type sum<std::string>.

When running this code, it will print 10 as the numeric sum and abcdef as the string sum.