Why Does C `constexpr` Prevent Undefined Behavior at Compile Time?

Why does constexpr exclude undefined behavior?

The C standard requires that constexpr expressions must not involve undefined behaviors. This allows compilers to prevent errors or unexpected results that could arise from evaluating expressions with undefined behaviors during compilation.

Advantages of this Exclusion

By excluding undefined behaviors, the compiler can:

• Detect errors early: Expressions with undefined behaviors will result in compile-time errors, even if they might not cause errors at runtime.
• Guarantee consistent results: The exclusion ensures that constexpr expressions always return well-defined values, eliminating the possibility of unexpected outcomes.
• Enable SFINAE: This exclusion allows programmers to use SFINAE (Substitution Failure Is Not An Error) to determine whether an expression would result in undefined behavior at compile time.

Specific Benefits

The exclusion of undefined behaviors in constexpr expressions is particularly useful for the following scenarios:

• Preventing integer overflows during addition, which would otherwise result in a non-constant value.
• Detecting undefined behavior in arrays or pointers, such as accessing elements beyond the bounds.
• Identifying certain shift operations that exceed the width of the data type.

Supporting Evidence

The following code examples demonstrate the exclusion of undefined behaviors in constexpr expressions:

// Integer overflow (undefined behavior) not allowed in constexpr
constexpr int x = std::numeric_limits<int>::max() + 1; // Error

// Pointer arithmetic (undefined behavior) not allowed in constexpr
constexpr int *ptr = &x[20]; // Error

// Shift operation (undefined behavior) not allowed in constexpr
constexpr int y = 1 << 33; // Error (assuming 32-bit int)

However, it's important to note that different compilers may handle certain types of undefined behaviors differently, particularly for shift operations.

The above is the detailed content of Why Does C `constexpr` Prevent Undefined Behavior at Compile Time?. For more information, please follow other related articles on the PHP Chinese website!