如何实现模板方法的自动类型转换？

Question

I am using self-written template methods to load and save settings from/into Qt's QSettings . QSettings receive/return data of type QVariant and QVariant has constructors taking basic types like int and double , but std::string .

My template method can therefore easily be used for int and double , but when using std::string I get compile error messages that there is no matching function call, because there is no known conversion from std::__cxx11::basic_string<char> to the types accepted by QVariant 's constructors.

Of course I can solve the problem by writing a specialization of my template method. Still, I am hesitating to do that, because there is exactly little call that causes the problem. The rest of the code is just fine for all types.

Is it possible to provide a user-defined type conversion such that the compiler can automatically use for type-conversion when compiling the template method?

Here is a minimal example that tries to reproduce the situation without Qt classes:

#include <string>
#include <vector>

class A
{
public:

    A(int)
    {
    }

    A(double)
    {
    }
};

std::vector<A> globalStorage;

template <typename Type>
void store(Type element)
{
    // A lot of lengthy storage preparation code
    // ...
    //

    // Final Storage
    globalStorage.push_back(A(element));
}

// TODO: A specialization of the template method solves the problem. Is there
// another way to provide a user-defined type conversion?
template <>
void store<std::string>(std::string element)
{
    // A lot of lengthy storage preparation code
    // ...
    //

    globalStorage.push_back(A(std::stoi(element)));
}

int main()
{
    double number1 = 1.0;
    int number2 = 2.0;
    float number3 = 3.0;

    store (number1);
    store (number2);
    store (number3);

    std::string number4 = "4";
    store(number4);

    return 0;
}

Answer 1

Create an overloaded non-template for std::string that forwards to your method.

template <typename Type>
void store(Type element)
{
    // A lot of lengthy storage preparation code
    // ...
    //

    // Final Storage
    globalStorage.push_back(A(element));
}

void store(const std::string& s) {
    //convert s to QString or char* or a number, as you want
    auto converted_element = ...;
    store(converted_element);
}