Templates inferring type T from return type

问题

I have a template as follows:

template <class T>
vector<T> read_vector(int day)
{
  vector<T> the_vector;
  {...}
  return the_vector;
}

I would like to be able to do something like

vector<int> ints = read_vector(3);
vector<double> doubles = read_vector(4);

Is it possible for C++ templates to infer the return type from when they're called, or should I just pass a dummy argument to the template with the type I want to the vector to have? The latter works but is messier.

回答1:

#include <vector>

struct read_vector
{
    int day;
    explicit read_vector(int day) : day(day) {}

    template <typename T, typename A>  
    operator std::vector<T, A>()
    {
        std::vector<T, A> v;
        //...
        return v;
    }
};

int main()
{
    std::vector<int> ints = read_vector(3);
    std::vector<double> doubles = read_vector(4);
}

DEMO

来源：https://stackoverflow.com/questions/53563499/templates-inferring-type-t-from-return-type