Explicit This Member Function: A Tale of Compiler Differences

Have you ever stumbled upon a piece of code that compiles smoothly with Microsoft Visual Studio (MSVC) but throws errors with Clang and GCC? You’re not alone! In this article, we’ll delve into the intricacies of the “explicit this member function” and explore why MSVC accepts it while Clang and GCC reject it. Buckle up, folks, and get ready to learn about the subtle nuances of C++ compiler behavior!

The Mysterious Case of the Explicit This Member Function

Let’s start with a simple example that demonstrates the issue:

class MyClass {
public:
    void myFunction() { /* some code */ }

    explicit MyClass::MyClass() { /* some code */ }
};

This code might look innocent, but it’s the source of our compiler woes. The `explicit MyClass::MyClass()` function is the culprit, and we’ll see why in just a moment.

What’s the purpose of the `explicit` keyword?

The `explicit` keyword is used to specify that a constructor or conversion function should not be used for implicit conversions. In other words, it’s a way to prevent the compiler from automatically converting one type to another without an explicit cast.

In the context of constructors, `explicit` ensures that the compiler doesn’t use the constructor for implicit conversions. For example:

class MyClass {
public:
    explicit MyClass(int x) : x_(x) {}
    int getX() { return x_; }
private:
    int x_;
};

MyClass obj = 5;  // Error: cannot convert 'int' to 'MyClass' in initialization

In this example, the `explicit` constructor prevents the compiler from implicitly converting the `int` value `5` to a `MyClass` object.

The MSVC Exception: Why It Compiles

So, why does MSVC accept the `explicit MyClass::MyClass()` function? The answer lies in the way MSVC handles explicitly qualified names.

In C++, when you use an explicitly qualified name (e.g., `MyClass::MyClass()`), the compiler treats it as a non-static member function. This means that the `this` pointer is implicitly passed as the first argument, just like with any non-static member function.

In the case of the `explicit MyClass::MyClass()` function, MSVC interprets it as a non-static member function, which allows it to compile without issues. However, this behavior is non-standard and specific to MSVC.

The Clang and GCC Verdict: Why It Fails to Compile

Now, let’s see why Clang and GCC reject the `explicit MyClass::MyClass()` function.

According to the C++ standard (section 9.3.3.1 [class.conv.ctor] paragraph 1), a constructor cannot be declared `explicit` if it’s not a converting constructor. A converting constructor is a constructor that can be called with a single argument, such as `MyClass(int x)`. Since our `explicit MyClass::MyClass()` function doesn’t meet this criterion, it’s not a valid use of the `explicit` keyword.

Clang and GCC adhere to the C++ standard more strictly than MSVC, which means they reject the `explicit MyClass::MyClass()` function as invalid.

What’s the Solution? How to Make It Work

So, what can you do if you need to use the `explicit` keyword with a constructor? The solution is straightforward: remove the explicitly qualified name and use a simple constructor declaration:

class MyClass {
public:
    explicit MyClass() { /* some code */ }
};

This code will compile without issues on all three compilers: MSVC, Clang, and GCC.

Best Practices: Avoiding the Pitfalls

To avoid compiler-specific issues like this one, follow these best practices:

• Stick to the C++ standard: When in doubt, consult the C++ standard documentation to ensure your code is compliant.
• Test on multiple compilers: Verify that your code compiles and runs correctly on different compilers, including MSVC, Clang, and GCC.
• Avoid non-standard extensions: Be cautious when using compiler-specific extensions, as they might not be portable across different platforms.

Conclusion: The Importance of Understanding Compiler Behavior

In conclusion, the `explicit this member function` issue serves as a reminder of the importance of understanding compiler behavior and adhering to the C++ standard. By being aware of the differences between MSVC, Clang, and GCC, you can write more portable, reliable code that compiles and runs smoothly on a wide range of platforms.

Remember, knowledge is power, and in the world of C++ development, understanding compiler behavior is crucial for producing high-quality software.

Compiler	Behavior
MSVC	Accepts explicit this member function
Clang	Rejects explicit this member function
GCC	Rejects explicit this member function

In this table, we summarize the behavior of each compiler when faced with the `explicit this member function` issue. Remember to keep this in mind when writing cross-platform C++ code.

Final Thoughts

The world of C++ development is full of intricacies and subtleties. By understanding the nuances of compiler behavior, you can write better code, avoid common pitfalls, and create software that stands the test of time.

Stay curious, keep learning, and always keep the C++ standard close at hand!

Frequently Asked Question

Get the inside scoop on why explicit member function use is accepted by MSVC but rejected by Clang and GCC!