关于C++ Lambda捕获私有类成员及相关技术点的疑问

Hey there, let's break down the tricky parts of this code that have you confused! First, let's restate the code snippet clearly to make our analysis easier:

//... 
bool dump_dvars() { 
    fstream& refvar{ output_file }; 
    for_each(_array_start, _array_start + *_array_size, 
        [&refvar](const void* dvar) -> void { 
            //work with output_file 
        });
    //...
}

1. Lambda Captures & Accessing Private Class Members #

The [&refvar] part is the capture list of the lambda expression, and here it's using reference capture:

• The & means we're capturing a reference to refvar, so the lambda uses the exact same object as the outer refvar (which is just a reference to output_file). Any operations on refvar inside the lambda directly affect output_file.
• As for accessing the private member output_file: since dump_dvars() is a member function of the class, it already has full access to the class's private members. By creating refvar as a reference to output_file, we're just passing that access through to the lambda—there's no "breaking" of private access rules here.

If you want a more direct approach, you could capture the this pointer instead:

[this](const void* dvar) -> void {
    // Now you can use this->output_file directly here!
}

This works because capturing this gives the lambda access to all the class's members (public and private) just like any other member function.

2. What's const void* dvar? #

The const void* is a generic, read-only pointer, and it's perfect for this use case:

• void* is a type-agnostic pointer—it can point to any data type (int, float, custom structs, you name it). This makes sense if _array_start points to an array that holds mixed or unknown types; using void* lets us handle every element's address uniformly.
• The const keyword ensures we can't modify the data that dvar points to inside the lambda. It's a safety measure to prevent accidental changes to the original array data.
• To actually use the data, you'll need to cast dvar back to its original type with static_cast. For example, if the array holds integers:

[this](const void* dvar) -> void {
    const int* int_value = static_cast<const int*>(dvar);
    this->output_file << *int_value << "\n";
}

3. A Quick Note on std::for_each #

The for_each function is an STL algorithm that iterates over every element in the range [_array_start, _array_start + *_array_size). For each element, it passes the element's address to the lambda. It's a clean, idiomatic alternative to writing a raw for loop for traversal.

内容的提问来源于stack exchange，提问作者Jack Of Blades