Vector parameter in a function doesn't seem to actually apply to input?-CodePudding

I have a vector variable named intVec, and I have a function named pushBack, that accepts a vector of type integer just like intVec, but when I actually pass that vector into the function in order to push_back the x parameter, nothing seems to happen.

Output expected from intVec.size() is 1

Output given from intVec.size() is 0

I'm genuinely confused as to what I'm doing incorrectly here.

Perhaps I'm missing something extremely obvious.

#include <vector>

std::vector<int> intVec;

void pushBack(int x, std::vector<int> vec) {
    vec.push_back(x);
}

int main() {
    pushBack(10, intVec);
    std::cout << intVec.size();
}

CodePudding user response：

That is because you pass the vector by value instead of by reference (or pointer).

This means, that when you call the function, the vector you call 'push_back' on is actually an independent copy of the original vector, which get deallocated upon leaving the function.

Try this header instead:

void pushBack(int x, std::vector<int> &vec) {
    vec.push_back(x);
}

The & tells the compiler to pass by reference, meaning that whatever you do to vec, you also do to the vector you originally passed.

CodePudding user response：

Write pushBack as below:

void pushBack(int x, std::vector<int>& vec) {
    vec.push_back(x);
}

The only difference is that here vec is being passed by reference. In your code it is being passed by value.