Is \"Want Speed? Pass by Value\" Always True: When Does Passing by Reference Outperform Passing by Value?

"Want Speed? Pass by Value" and Performance Optimization

When dealing with copy-heavy operations, developers often strive to optimize performance. This premise has led to the axiom "Want speed? Pass by value" coined by Scott Meyers. However, this notion raises the question: does passing by value always provide a performance advantage over passing by reference?

Considering the following classes 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& name) : mem_name(name) {}
};</code>

In X, the constructor takes a copy of the argument and uses the move constructor to initialize its member variable mem_name. In Y, the constructor takes a const reference and initializes mem_name directly from the argument.

Now, let's examine a scenario where we utilize these classes:

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

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

Function foo() returns a temporary value used to initialize name in X and mem_name in Y.

In the case of X, the compiler can optimize the construction of foo() and place its return value directly into name. Afterward, it moves name into mem_name. This process results in a single move, without any copy.

In contrast, Y cannot perform this optimization. It binds the temporary return value of foo() to its reference name and then copies that value into mem_name. Therefore, Y performs a copy.

In summary, when passing an rvalue (temporary object), passing by value in X has the potential to optimize the process to a single move, while passing by reference in Y requires a copy. However, it's important to note that this optimization depends on compiler capabilities, and profiling is always advisable to determine the actual performance impact.

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