[[nodiscard]] in C++ function declarations: Demystifying the consequences of ignoring return values

The

[[nodiscard]] attribute indicates that the return value of the function must not be ignored, otherwise it will cause a compiler warning or error to prevent the following consequences: uninitialized exceptions, memory leaks, and incorrect calculation results.

[[nodiscard]] in C function declaration: Demystifying the consequences of ignoring return values

Introduction

In C programming, the [[nodiscard]] attribute flag indicates that the return value of a function cannot be ignored. However, the consequences of ignoring this property are less well known. This article will delve into the role of [[nodiscard]] and demonstrate the potential pitfalls of ignoring return values ​​through practical examples.

What [[nodiscard]] does

[[nodiscard]] attribute instructs the compiler that the return value of a function is an important value and should not be discarded. The compiler will issue a warning or error when the return value of a function with the [[nodiscard]] attribute is not used. This helps prevent valuable calculation results from being accidentally discarded.

Practical case

Consider the following function with the [[nodiscard]] attribute:

[[nodiscard]] int CalculateArea(int width, int height)
{
    return width * height;
}

If we call the CalculateArea function and ignore its return value , the compiler will generate a warning:

int main()
{
    CalculateArea(10, 5); // 编译器警告：丢弃了带有 [[nodiscard]] 函数的返回值
    return 0;
}

However, if we use the return value, the compiler will not issue a warning:

int main()
{
    int area = CalculateArea(10, 5); // 正确：使用了 [[nodiscard]] 函数的返回值
    return 0;
}

Consequences

Ignoring the return value of a function with the [[nodiscard]] attribute may result in the following consequences:

• Uninitialized exception: If the function returns a value that is used to initialize a variable, and If a variable is not initialized correctly, an uninitialized exception may occur.
• Memory leak: If a function returns a pointer to memory that should be freed, ignoring the return value can cause a memory leak.
• Incorrect calculation results: If the function returns a value that is used in subsequent calculations, ignoring the return value will result in incorrect calculation results.

Conclusion

[[nodiscard]] attributes are critical to ensuring code robustness. By using [[nodiscard]], programmers can prevent the potential pitfalls of ignoring the return value of a function with the [[nodiscard]] attribute.

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