CODEWITHSUNDEEP
c++functionvariablesarguments
user15747807
user15747807

Reputation:

C++ difference between passing argument from a function call or passing argument from variable

Whats the difference between this:

function1(function2());

And this:

var1 = function2();
function1(var1);

In terms of efficiency or whatever, what is the best option?

Upvotes: 0

Views: 96

Answers (1)

463035818_is_not_an_ai
463035818_is_not_an_ai

Reputation: 122486

Before C++11 there is no big difference. Since move semantics were introduced the difference can be substantial. For example when a function needs to make a copy of its parameter it can have two overloads: one that actually does make a copy and another one that can move when the parameter is a temporary:

#include <iostream>

struct bar {};
bar b() { return {};}

struct foo {
    bar b;
    void set(const bar& in) { 
        std::cout << "copy \n";
        b = in;
    }
    void set(bar&& in){
        std::cout << "no need to copy, can move\n"; 
        b = std::move(in);
    }
};

int main() {
    foo f;
    bar b1;
    f.set(b1);
    f.set(b());
}

bar in this example is just a lightweight class, but when it is cheap to move but expensive to copy then f.set(b1) is less efficient than f.set(b()). The caller could f.set(std::move(b1)), but it is more clear to call f.set(b()) rather than having a moved from object hanging around.

However, already before C++11, the question you should actually ask is: Does it make sense to give a name to the result of calling function2? Do you need the result only to call function1 or do you need it also elsewhere? And this can be answered independently of whether you are writing ancient C++ or whether move semantics are involved. In short: Write code to be clear and readable. Concerns about efficieny are for a later stage, when you have correct and working code that you can measure and profile.

Upvotes: 2

Related Questions