What is the difference between STL function objects and C++ lambda expressions?

Function objects and lambda expressions are both tools for creating anonymous functions. The main differences are: Syntax: Function objects use class definitions, while lambda expressions use [] syntax. Scope: Function objects can be used outside the class, whereas lambda expressions are limited to the scope of definition. Capture: Function objects cannot capture external variables, but lambda expressions can capture through capture lists. Overhead: Function object creation overhead is low, lambda expression overhead is high. Reusability: Function objects are usually reusable, lambda expressions are usually single-use.

The difference between STL function objects and C lambda expressions

Function objects and lambda expressions are both used to create in C Tools for anonymous functions. While they have similarities, there are also key differences:

Syntax

• Function objects: Use classes to declare.
• lambda expression: Use [] syntax definition.

Scope

• Function object: Can be used outside the class.
• lambda expressions: are only valid within the scope in which they are defined.

Capture

• Function object: Cannot capture external variables.
• lambda expression: External variables can be captured through the capture list.

Overhead

• Function object: Low creation overhead.
• lambda expression: High overhead to create because it requires generating a closure.

Reusability

• Function objects: Usually reusable.
• lambda expression: Usually a one-time use.

Practical Case

Suppose we have an array of integers and we want to find the first element that meets a certain condition (for example, the first one is greater than 10 Elements).

Using a function object:

class GreaterThan10 {
public:
    bool operator()(int x) { return x > 10; }
};

int main() {
    int arr[] = {1, 5, 7, 12, 14};
    auto found = find_if(begin(arr), end(arr), GreaterThan10());
    if (found != end(arr)) {
        cout << "First number greater than 10: " << *found << endl;
    }
    return 0;
}

Using a lambda expression:

int main() {
    int arr[] = {1, 5, 7, 12, 14};
    auto found = find_if(begin(arr), end(arr), [](int x) { return x > 10; });
    if (found != end(arr)) {
        cout << "First number greater than 10: " << *found << endl;
    }
    return 0;
}

In this example, the lambda expression is Create an anonymous function that determines whether the integer is greater than 10.

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