Lære Perfect Forwarding and std::forward

Perfect forwarding is a technique in template programming that preserves the value category (lvalue or rvalue) of function arguments. std::forward is used to achieve this, enabling efficient and correct resource transfers.

main.cpp


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#include <iostream>
#include <utility>
#include <string>

void process(const std::string& s)
{
    std::cout << "Lvalue processed: " << s << '\n';
}

void process(std::string&& s)
{
    std::cout << "Rvalue processed: " << s << '\n';
}

template<typename T>
void wrapper(T&& arg) { process(std::forward<T>(arg)); }

int main()
{
    std::string hello = "Hello";
    wrapper(hello);              // Passes lvalue
    wrapper(std::string("Hi"));  // Passes rvalue
}

In the code above, std::forward ensures that lvalues are passed as lvalues and rvalues as rvalues, allowing move semantics to work correctly in generic code.

Perfect forwarding is essential when writing wrapper or factory functions that pass arguments to other functions or constructors without losing their value category.

Note

std::move always casts to rvalue, which can lead to unintended moves of lvalues. std::forward preserves the original value category.

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