What are the restrictions on C++ functions returning lambda expressions?

Answer: C functions can return Lambda expressions, but there are the following limitations: Limitations: Lambda expressions should capture storage types (Captures by Value) Lambda expressions cannot return local variables Lambda expressions cannot return Lambda expressions

Restrictions when C functions return Lambda expressions

Lambda expressions are anonymous functions in C that capture variables from the surrounding context. Typically, functions can return other functions, including lambda expressions. However, there are some limitations to be aware of:

Lambda expressions should capture storage types (Captures by Value)

The returned Lambda expression cannot capture variables in the surrounding context citation. Instead, it must capture them based on copies in the capture list. For example:

int num = 0;

auto lambda = [num]() { 
    // num 被捕获为值，因此无法修改外部 num
    return num; 
};

Lambda expression cannot return local variables

The type returned by Lambda expression cannot be a local variable within a function. This means that the address or reference of a local variable cannot be returned. For example:

int main() {
    int num = 0;
    
    [[maybe_unused]] auto lambda = [&num]() { 
        return # 
    }; // 错误：返回局部变量 (&num)
}

Lambda expression cannot return Lambda expression

Lambda expression cannot return another Lambda expression. That is, you cannot return a Lambda expression that returns a Lambda expression. For example:

auto lambda1 = []() { 
    return [num](){}; 
}; // 错误：返回 Lambda 表达式

Practical case: Creating a sorting function

The following example shows how to use Lambda expressions in a C function to create a sorting function:

#include <algorithm>
#include <vector>

std::vector<int> Sort(const std::vector<int>& numbers) {
    std::sort(numbers.begin(), numbers.end(),
        [](const int& a, const int& b) { 
            return a < b; 
        });
    return numbers;
}

In this case, the Sort function returns a Lambda expression that is used as the comparison function for the std::sort function. It is important to note that the Lambda expression captures a copy of the numbers container, so its elements can be accessed safely during sorting.

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