The Mysterious Case of Constexpr Variables and Lambda Functions

Are you a seasoned C++ developer who has stumbled upon a peculiar issue with constexpr variables and lambda functions? Do you find yourself scratching your head, wondering why Visual C++ is throwing a compile error, while GCC is compiling just fine? Well, wonder no more! In this article, we’ll delve into the world of constexpr variables and lambda functions, exploring the intricacies of their interaction and providing clear instructions on how to navigate this seemingly baffling scenario.

The Setup

Let’s start with a simple example. Suppose we have a lambda function that returns a constexpr variable:

auto lambda = []() constexpr {
    return 42;
};

No issues here, right? We can happily get the constexpr variable from the lambda function:

constexpr int value = lambda();

This code compiles and runs smoothly on both Visual C++ and GCC. But, what happens when we try to create a new lambda function within another lambda function and attempt to get the constexpr variable from it?

The Problem

Here’s an example that illustrates the issue:

auto outer_lambda = []() {
    auto inner_lambda = []() constexpr {
        return 42;
    };
    constexpr int value = inner_lambda(); // Error in Visual C++
    return value;
};

Boom! Visual C++ throws a compile error, complaining that the constexpr variable cannot be initialized with a non-constexpr expression. But, wait, GCC compiles this code just fine! What’s going on here?

The Reason Behind the Error

The key to understanding this issue lies in the way constexpr variables are evaluated and the lifetime of lambda functions. When we define a constexpr variable, the compiler must be able to evaluate its value at compile-time. In the first example, the lambda function is defined at the top-level scope, and the constexpr variable is evaluated directly from the lambda function’s return value.

However, when we define a lambda function within another lambda function, things get more complicated. The inner lambda function is not a static entity; it’s an object that’s created on the fly when the outer lambda function is executed. This means that the inner lambda function’s lifetime is tied to the outer lambda function’s execution.

In Visual C++, the compiler is more conservative when it comes to evaluating constexpr variables. It requires that the evaluation of the constexpr variable be possible at compile-time, without relying on the execution of any code. Since the inner lambda function’s lifetime is tied to the outer lambda function’s execution, the compiler cannot guarantee that the constexpr variable can be evaluated at compile-time.

On the other hand, GCC takes a more relaxed approach, allowing the evaluation of constexpr variables to be delayed until the inner lambda function is actually executed. This is why GCC compiles the code successfully.

The Solution

So, how can we get around this issue in Visual C++? One way to solve this problem is to use a technique called “immediate function” or “immediate invocation.” We can define the inner lambda function and immediately invoke it, like this:

auto outer_lambda = []() {
    constexpr int value = ([]() constexpr {
        return 42;
    })(); // Immediate invocation
    return value;
};

By immediately invoking the inner lambda function, we ensure that the constexpr variable is evaluated at the point of invocation, which is within the outer lambda function’s scope. This allows Visual C++ to compile the code successfully.

Conclusion

In conclusion, the interaction between constexpr variables and lambda functions can be tricky, especially when dealing with nested lambda functions. While GCC may compile the code successfully, Visual C++ is more stringent in its evaluation of constexpr variables. By understanding the underlying reasons behind the error and using techniques like immediate function invocation, we can successfully get constexpr variables from lambda functions, even in the presence of nested lambda functions.

Best Practices

To avoid running into this issue, follow these best practices:

• Avoid defining constexpr variables within nested lambda functions.
• Use immediate function invocation to ensure that constexpr variables are evaluated at the point of invocation.
• Test your code on multiple compilers to ensure portability.

Common Scenarios

Here are some common scenarios where you might encounter this issue:

Scenario	Description
Nested algorithms	When implementing nested algorithms using lambda functions, you might need to get constexpr variables from inner lambda functions.
Template metaprogramming	In template metaprogramming, you might use lambda functions to compute values at compile-time. Getting constexpr variables from these lambda functions can be challenging.
Expression SFINAE	When using expression SFINAE, you might need to get constexpr variables from lambda functions to enable or disable overloads.

Further Reading

If you’re interested in learning more about constexpr variables, lambda functions, and their interactions, here are some recommended resources:

1. cppreference.com: constexpr
2. cppreference.com: Lambda expressions
3. Stack Overflow: Constexpr and lambda functions

By understanding the intricacies of constexpr variables and lambda functions, you’ll become a master of compile-time evaluation and be able to tackle even the most complex C++ scenarios.

Happy coding!

Frequently Asked Question

Ever wondered why getting a `constexpr` variable from a lambda function is a breeze with gcc, but Visual C++ throws a tantrum? Let’s dive into the whys and hows!