How do I set parameter default values that rely on other parameters?

问题

The following code compiles and works as expected.

#include <vector>

void function(std::vector<int> vec, int size=1);

int main(){
    std::vector<int> vec = {1,2,3};
    function(vec);
}


void function(std::vector<int> vec, int size){
    //code..
    return;
}

However, I would like the size parameter's default value to be deduced based on a previous parameter. So for example:

void function(std::vector<int> vec, int size=vec.size());

But this however results in:
error: local variable ‘vec’ may not appear in this context

Which doesn't surprise me; I assume it needs to know the default value at compile time. So do I make the function templated? How would I make the function templated in such a way that the parameter is still an int, and the vector, is still a vector of ints, and the size parameter is deduced to be the passed in vector's size by default.

I would not like to have to pass in the size of the vector during the function call.

回答1:

If you truly want a function with a size parameter, and want the size to default to the vector size (instead of just getting it as a local variable), then you could solve this with two functions. One taking just the vector, and one taking the vector and the size (without default). Then the one-argument function can just call the second.

void function(const std::vector<int>& vec, const size_t size)
{
   ...
}

void function(const std::vector<int>& vec)
{
   function(vec, vec.size());
}

回答2:

Why do you need second parameter if it based on the first?

    void function(std::vector<int> vec ){
        size_t size = vec.size();
        //code..
        return;
    }

Isn't it easier?

来源：https://stackoverflow.com/questions/19830677/how-do-i-set-parameter-default-values-that-rely-on-other-parameters

标签

c++

c++11

parameters

parameter-passing

default-value