Is Passing by Value Always Slower Than Passing by Reference? Unpacking the \'Want Speed? Pass by Value\' Debate

"Want Speed? Pass by Value": A Detailed Explanation

The concept of "Want speed? Pass by value" has sparked discussions in the programming community. This article delves into the performance implications of passing arguments by value versus reference, illuminating the circumstances under which passing by value can enhance speed.

Consider the following code snippets, depicting structures X and Y:

<code class="cpp">struct X {
    std::string mem_name;

    X(std::string name)
        : mem_name(std::move(name)) {}
};

struct Y {
    std::string mem_name;

    Y(const std::string &amp;name)
        : mem_name(name) {}
};</code>

In X's constructor, the argument "name" is copied and then moved into "mem_name." In Y's constructor, "name" is a const reference, eliminating the need for an initial copy. It might seem that Y's approach is more efficient, with only one copy operation.

However, the concept of "Want speed? Pass by value" underscores that in some scenarios, the copy can be omitted. Consider the following code:

<code class="cpp">std::string foo() { return "a" + std::string("b"); }

int main() {
    X(foo());
    Y(foo());
}</code>

In this case, X can construct the return value of foo() directly in the space of the "name" object, eliminating the need for an additional copy. Y, however, must copy the temporary return value into "mem_name." Thus, for rvalue arguments, passing by value (in this case, through X) can potentially result in a single move operation, while passing by reference (through Y) involves an unavoidable copy.

In summary:

• For lvalue arguments, both X and Y perform one copy operation.
• For rvalue arguments, X can potentially perform a single move operation, while Y must always perform a copy.

While passing by reference typically translates to passing a pointer, a move operation can be operationally faster than passing a pointer. Therefore, passing by value (in some cases) can avoid unnecessary copies and enhance performance.

It is important to note that these guidelines are not absolute and should be interpreted with discretion. In cases of doubt, profiling is recommended to determine the actual performance impact for a specific scenario.

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