C ++函数的可选std :: vector参数

Question

I have seen some answers here, but they may not apply here. I have a (member) function that is mostly used with only one argument (the 1st):

const std::complex<double> Class::func(const std::complex<double> &x, \
                                       std::vector<double> &y = 0, \
                                       std::vector<double> &z = 0) const;

I would like for y and z to be optional, maybe even based on a fourth argument of type string , something like this: func( , , , const std::string &choice) , but which would let me only pass one argument to the function and the other two not be used. If I do pass y , for example, its declaration would have to be done previous of calling func() , same for z , but I'd like this to be optional, the function can be a bit heavy on the math side and there's no need to add the burden of calculating two additional vectors if they're not needed. Is this possible?

Answer 1

Use pointers and pass a nullptr , check for this..

const std::complex<double> Class::func(const std::complex<double> &x, \
                                       std::vector<double> *y = nullptr, \
                                       std::vector<double> *z = nullptr) const;

I've defaulted them here, but that's optional...

Answer 2

Given the use of the std::vector however, what does an "empty" vector mean? This could be used as the "optional" value you require.

Some notes on the original code; the "null" reference is not valid, ie std::vector<double> &y = 0 does not compile. A pointer could be used in it place, it would not immediately mean allocation are required.

const std::complex<double> Class::func(
    const std::complex<double> &x,
    std::vector<double>* y = nullptr,
    std::vector<double>* z = nullptr) const;

Function overloads are often an answer for situation such as this; the internals of the functions are defer to each for the final implementation...

const std::complex<double> Class::func(
    const std::complex<double> &x) const;
const std::complex<double> Class::func(
    const std::complex<double> &x,
    std::vector<double>& y) const;
const std::complex<double> Class::func(
    const std::complex<double> &x,
    std::vector<double>& y,
    std::vector<double>& z) const;

There is also std::optional .

const std::complex<double> Class::func(
    const std::complex<double> &x,
    std::optional<std::vector<double>> &y,
    std::optional<std::vector<double>> &z) const;

If not available in your standard library, there is also boost::optional .

Answer 3

References cannot be optional but there is something called boost::optional<>

If you are concerned about copy-operations you should use a pointer instead and use nullptr as default.

If you also want to make it dependent on a string at the end you could write a wrapper function

Just add another function:

const std::complex<double> Class::func(
    const std::complex<double> &x,
    std::string str,
    std::vector<double> *y = nullptr,
    std::vector<double> *z = nullptr) const
{
    if(str == ...)
        return func(....);
    else
       return func(....); //different call
}