C ++：用户定义类型的向量？

Question

I have a vector that I'm storing data in. I want users to be able to decide the level of precision for the storage vector (either floats or doubles).

How do I declare a vector based on user input? This obviously doesn't work:

std::vector<userWantsFloats ? float : double> data; ...

Answer 1

You cannot choose the type used in a class template instantiation based on a value that is known only at run time.

Use of

std::vector<userWantsFloats ? float : double> data;

is OK if the value of userWantsFloats is known at compile time. It is not OK if the value of userWantsFloats is not known only at run time.

You'll have to use something along the lines of:

if ( userWantsFloats )
{
   std::vector<float> data;
   // Use data
}
else
{
   std::vector<double> data;
   // Use data
}

To be able to maximize reuse of rest of your code, they have to be function templates and/or class templates.

template <typename T>
void myAppLogic(std::vector<T>& data)
{
   // Do the work of your application
}

if ( userWantsFloats )
{
   std::vector<float> data;
   myAppLogic(data);
}
else
{
   std::vector<double> data;
   myAppLogic(data);
}

Answer 2

A c++17 option could be to use a std::variant .

std::variant<std::vector<float>, std::vector<double>> data;

You then initialize the variant based on user input, but can write any logic only once by using std::visit

std::visit([](auto&& vec) {
  // Do your thing
}, data);

If C++17 is not an option, then boost has a variant template that inspired the now standard one. You can use that instead. Consult the boost documentation on the subject.