Which is faster? Pass by reference vs pass by value C++

问题

I thought that pass by reference should be faster then pass by value because the computer isn't copying data, it just points to the address of data.

But, consider the following C++ code:

#include <iostream>
#include <cassert>
#include <cmath>

using namespace std;

// do not use pass by reference& with this function, it will become so slow
unsigned long long computePascal(const unsigned int row, const unsigned int position) {
    if (position == 1 || position == row)
    return 1L;
    unsigned long long left = computePascal(row-1, position-1);
    unsigned long long right = computePascal(row-1, position);
    unsigned long long sum = left + right;
    return sum;
}

int main() {
    int row, position;
    cout << "Input row and position: ";
    cin >> row >> position;
    assert(position > 0 && position <= row);
    unsigned long long pascalNumber = computePascal(row, position);
    cout << pascalNumber << endl;
    return 0;
}

Now this code is just a normal program that compute pascal number recursively by entering the desired row and position of the triangle.

I tried putting row 50 and position 7 and it computes around 1 second (pass by value). The output is about 13 million which is correct. So I thought that it could be faster if I pass by reference instead because it doesn't need to copy a lot of data. But this is very wrong and I don't know why it took longer time about 3 times the amount it took for passing by value. The question is ...

Why this program computes so slow after I try changing it to pass by const reference?

Does it matter that because recursive function is an exception that you should pass by value rather than const reference?

回答1:

The answer to "Which is faster" is usually "It depends".

If instead of passing four bytes of data you are passing an eight byte pointer to data, then you can't really expect that to make things faster. If instead of passing 100 bytes of data you are passing an eight byte pointer to data, that's different.

But now the function doesn't have the data, it only has a reference. So whenever it needs to read the data, it has to do that indirectly through the reference. That takes longer. If you pass a 100 byte object and only read eight byte of it, you still are likely to win. But if you actually read all the data, and maybe multiple times, then it could easily be faster to pass the value even for large objects.

The real difference comes when you pass an object, and passing by value means a more or less complex constructor will be called. Passing by reference means no constructor. But int has no constructor anyway.

And then there is optimisation. Passing by value means the compiler knows your function is the only one with access to the data. Pass by reference means the data could be anywhere. If you have two int& parameters, I could pass the some int twice. So increasing row might increase pos. Or it might not. That kills optimisations.

And then there is the rule of optimisation: "Measure it". You measured it and found what's faster. Sometimes things are faster or slower for no good reason whatsoever.

来源：https://stackoverflow.com/questions/26552481/which-is-faster-pass-by-reference-vs-pass-by-value-c