Extern Constexpr in Visual Studio

The /Zc:externConstexpr option in Visual Studio is a compiler flag that enforces standard-compliant behavior for constexpr variables declared as extern.

This option aligns Visual Studio with the C++ standard, which allows constexpr variables to be declared as extern.

Understanding extern constexpr

• Standard Compliance: According to the C++ standard, constexpr variables can be declared as extern, meaning their definition exists in another translation unit.
• Initialization Requirement: Despite being extern, these variables must be initialized because constexpr requires a value at compile time.

The Issue in Visual Studio

By default, Visual Studio may not allow extern constexpr variables, resulting in a compilation error:

1extern constexpr int Value{10};

Enabling /Zc:externConstexpr

To resolve this, you need to enable the /Zc:externConstexpr option, which makes Visual Studio adhere to the C++ standard.

Via Command Line

Add the flag directly in your compile command:

1cl /Zc:externConstexpr main.cpp

Via Visual Studio IDE

• Open your project properties.
• Navigate to Configuration Properties -> C/C++ -> Command Line.
• Add /Zc:externConstexpr to the Additional Options box.

Example

Let’s see a complete example of a program that uses a extern constexpr variable:

1// config.h
2#pragma once
3
4extern constexpr int MaxConnections;

1// config.cpp
2#include "config.h"
3
4constexpr int MaxConnections{10};

1// main.cpp
2#include <iostream>
3#include "config.h"
4
5int main() {
6  std::cout << "MaxConnections: "
7    << MaxConnections;
8}

We can now compile our project with the /Zc:externConstexpr flag, and then run it:

1cl /Zc:externConstexpr config.cpp main.cpp -o myProgram
2./myProgram

1MaxConnections: 10

Benefits of /Zc:externConstexpr

• Standard Compliance: Ensures your code adheres to the C++ standard, making it more portable and predictable.
• Consistent Behavior: Provides consistent behavior across different compilers and platforms.

Summary

• Purpose: The /Zc:externConstexpr option ensures that extern constexpr variables are treated according to the C++ standard.
• Usage: Enable this option in Visual Studio to avoid compilation errors related to extern constexpr variables.
• Benefit: It promotes code portability and standard compliance, ensuring your code works correctly across different compilers.

Using /Zc:externConstexpr in Visual Studio allows you to take full advantage of constexpr variables with extern linkage, adhering to the C++ standard and avoiding compiler-specific issues.