Using Lambdas with std::function

Yes, you can use lambda expressions with std::function. Lambda expressions offer a convenient way to create anonymous function objects that can be stored in std::function objects. However, there are a few important considerations and limitations to be aware of when using lambdas with std::function.

Capturing Variables

• Lambda expressions can capture variables from their surrounding scope by value or by reference. When a lambda is stored in a std::function, the captured variables are copied or moved into the std::function object, making it self-contained and allowing it to outlive the original scope of the captured variables.
• Caution: Capturing variables by reference requires careful management, as the references must remain valid for the lifetime of the std::function object.

Capturing this

• If you capture this in a lambda and store it in a std::function, ensure that the std::function object does not outlive the object that this points to.
• Tip: Capturing this by value ([this]) is safer than capturing it by reference ([&]), as it creates a copy of the this pointer.

Overloaded Function Call Operator

• If a lambda expression has an overloaded function call operator (i.e., multiple operator() with different signatures), you need to explicitly specify which overload should be used when storing it in a std::function.
• How: You can achieve this by casting the lambda to the appropriate function pointer type.

Example

Here's an example demonstrating these considerations:

1#include <functional>
2#include <iostream>
3
4class MyClass {
5 public:
6  void Method() {
7    auto lambda = [this] {
8      std::cout << "Lambda called on object: "
9        << this << "\n";
10    };
11
12    // Store the lambda in a std::function
13    std::function<void()> Callable = lambda;
14
15    // Call the lambda through the std::function
16    Callable();
17  }
18};
19
20int main() {
21  MyClass obj;
22  obj.Method();
23
24  // Lambda with overloaded function call operator
25  auto overloadedLambda = [](int x) {
26    std::cout << "Int overload: " << x << "\n";
27  };
28
29  // Calls the int overload
30  overloadedLambda.operator()(3);
31
32  // Store the lambda in a std::function,
33  // explicitly specifying the overload
34  std::function<void(int)> overloadedCallable{
35    static_cast<void (*)(int)>(overloadedLambda)};
36
37  overloadedCallable(42);
38}

1Lambda called on object: 0x7ffee2b6e738
2Int overload: 3
3Int overload: 42

In this example:

• The lambda captures this by value in MyClass::Method(), ensuring that the std::function object can safely outlive the MyClass object.
• The overloadedLambda has an overloaded function call operator. By explicitly casting it to a function pointer type void(*)(int), we specify which overload should be used when storing it in a std::function.

Conclusion

Using lambdas with std::function is straightforward, but it's crucial to be mindful of variable capture and lifetime issues, as well as overload resolution for lambdas with multiple operator() overloads.