What's the Difference Between `std::move` and `std::forward` in C ?

Unveiling the Distinctions: std::move vs. std::forward

In the context of C , std::move and std::forward play crucial roles in manipulating and forwarding objects. However, understanding their subtle differences can be challenging. This article aims to clarify these differences and provide insights on their usage.

std::move: Treating Objects as Rvalues

std::move takes an object and presents it as a temporary, meaning an rvalue. This allows the object to be perceived as a value that has just been constructed and can be safely destroyed or modified.

std::forward: Perfect Forwarding

std::forward, unlike std::move, has a narrower purpose. Its primary function is to cast a templated function parameter to the same value category (lvalue or rvalue) that the caller used to pass it in the function. This feature enables the seamless forwarding of rvalues as rvalues and lvalues as lvalues, a concept known as "perfect forwarding."

Key Differences

To highlight the distinction, let's investigate a code example:

void overloaded(const int &arg) { std::cout << "by lvalue\n"; }
void overloaded(int &&arg) { std::cout << "by rvalue\n"; }

template<typename T>
void forwarding(T &&arg) {
    std::cout << "via std::forward: ";
    overloaded(std::forward<T>(arg));
    std::cout << "via std::move: ";
    overloaded(std::move(arg));
    std::cout << "by simple passing: ";
    overloaded(arg);
}

int main() {
    std::cout << "initial caller passes rvalue:\n";
    forwarding(5);
    std::cout << "initial caller passes lvalue:\n";
    int x = 5;
    forwarding(x);
}

In this example, we define two overloaded functions (overloaded) for lvalues and rvalues, respectively. The forwarding function showcases the usage of both std::move and std::forward.

When passing an rvalue (line 14) to forwarding, both std::forward and std::move behave the same, forwarding it as an rvalue to the appropriate overload. However, with an lvalue (line 17), std::forward preserves the lvalue nature, while std::move would treat it as an rvalue.

When to Use std::move or std::forward

• Use std::move when you intend to take ownership of an argument or explicitly want an object to be treated as a temporary, especially in move operations.
• Use std::forward only when performing perfect forwarding of template function parameters to preserve the value category as defined by the caller. Otherwise, prefer static casts with clear documentation.

Comprehending these distinctions empowers developers to effectively manipulate and pass objects in C code, ensuring correct behavior and optimized performance.

The above is the detailed content of What's the Difference Between `std::move` and `std::forward` in C ?. For more information, please follow other related articles on the PHP Chinese website!