constexpr class member without assignment doesn’t compile on GCC, but does on Clang and MSVC

by

Summary

The issue at hand is a compiler discrepancy where a constexpr class member without an explicit assignment compiles on Clang and MSVC but fails to compile on GCC. This discrepancy arises from differences in how each compiler interprets the C++ standard regarding the initialization of constexpr static data members.

Root Cause

The root cause of this issue is the interpretation of the C++ standard by each compiler. Specifically, the standard requires that constexpr static data members must have an initializer. The key points are:

  • GCC strictly enforces this rule, requiring an explicit initializer (e.g., = {} or = TCP_SocketDeleter{}) for constexpr static data members.
  • Clang and MSVC seem to implicitly initialize the constexpr static data member with a default constructor if no initializer is provided, which aligns with a more lenient interpretation of the standard but is technically not compliant.

Why This Happens in Real Systems

This discrepancy can occur in real systems due to several reasons:

  • Compiler updates and versions: Different versions of compilers may handle constexpr static data members differently, leading to compilation issues when switching between compilers or updating compiler versions.
  • Code portability: Code that compiles without issues on one compiler may fail on another due to these discrepancies, highlighting the importance of testing code across multiple compilers.
  • Lack of explicit initialization: Developers might overlook the need for explicit initialization of constexpr static data members, especially if their code compiles without errors on their primary development compiler.

Real-World Impact

The real-world impact of this issue includes:

  • Compilation errors: The most immediate impact is the failure to compile code that is syntactically correct but does not meet the specific compiler’s requirements for constexpr static data members.
  • Portability issues: Code that works on one platform or compiler may not work on another, affecting the portability and reliability of software projects.
  • Development delays: Resolving these discrepancies can consume significant development time, especially in large, complex projects where tracking down the source of compilation errors can be challenging.

Example or Code

class TCPSocket {
public:
    struct TCP_SocketDeleter {
        void operator() (int*) const;
    };
    // Explicit initialization to comply with GCC
    constexpr static inline TCP_SocketDeleter socket_deleter = {};
};

How Senior Engineers Fix It

Senior engineers fix this issue by:

  • Explicitly initializing constexpr static data members to ensure compliance with the C++ standard and to avoid compiler discrepancies.
  • Testing code on multiple compilers to catch portability issues early in the development cycle.
  • Following best practices for C++ coding, including explicit initialization of static members and using compiler flags that enforce strict standard compliance.

Why Juniors Miss It

Junior engineers might miss this issue due to:

  • Lack of experience with the nuances of C++ and its various compilers.
  • Insufficient testing on different compilers, leading to a false sense of security if the code compiles on their primary development environment.
  • Overlooking the importance of explicit initialization for constexpr static data members, especially if their code compiles without errors on the compilers they commonly use.