C++ syntax error: Default parameters cannot be used in function parameters. What should I do?

C is a powerful programming language that is often used to develop various types of applications and software. However, during C programming, you may encounter situations where default parameters cannot be used in function parameters, which is usually caused by syntax errors. So, how to deal with this kind of grammatical error? This article will introduce some methods of handling errors to help you better understand C programming.

First, let’s understand what the default parameters are. In C, you can set default values ​​for parameters in the function definition. In this way, when we call the function, if the corresponding parameters are not passed, the compiler will use the default value. For example, the following code defines a function add(), in which parameters a and b have default values:

int add(int a = 0, int b = 0) {
    return a + b;
}

When calling the add() function, we can not pass parameters and use the default values:

int result = add(); // result = 0

But, how should we deal with the situation when default parameters cannot be used in function parameters? This usually includes the following situations:

Situation 1: Inconsistency between function declaration and function definition

In C, functions usually need to be declared and defined in both places, and the declaration is usually placed In header files, definitions are usually placed in the source file. If default parameters are set in the function declaration but not in the function definition, there will be a problem that the default parameters cannot be used in the parameters. For example:

// 声明
int add(int a, int b = 0);

// 定义
int add(int a, int b) {
    return a + b;
}

The parameter b in the declaration here has a default value, but the parameter b in the definition does not have a default value, a problem will occur. The solution is to add the default parameter in the definition:

// 声明
int add(int a, int b = 0);

// 定义
int add(int a, int b /*= 0*/) {
    return a + b;
}

Add a comment /= 0/ to parameter b in the definition, indicating that this parameter originally has a default value. But it is overloaded in the definition.

Case 2: Function overloading

Function overloading refers to the definition of multiple functions with the same name in the same scope, but their parameter lists are different. In the case of function overloading, if a default value is used in the parameters of a function, then the parameters of other functions with the same name must also be set to default values. For example:

int add(int a, int b) {
    return a + b;
}

int add(int a) {
    return a + 10;
}

int add(int a, int b = 0) {
    return a + b;
}

Three functions add() with the same name are defined here. The parameters of the second function have no default values, while the parameters of the other two functions use default values. This will cause the compiler to be unable to determine which function to call, resulting in an error that default parameters cannot be used in the parameters. The solution is to add default values ​​to the parameters of all functions with the same name:

int add(int a, int b = 0) {
    return a + b;
}

int add(int a = 0) {
    return a + 10;
}

int add(int a, int b = 0) {
    return a + b;
}

Here, a default value is also added to the parameter a of the second function, and the parameters of the first and third functions are also added. Default values ​​have also been added to .

Case 3: Function pointers and function references

In C, function pointers and function references may also cause errors that default parameters cannot be used in parameters. This is because when we call a function using a function pointer or function reference, the compiler cannot determine whether the function's parameters have default values. For example:

typedef int(*add_func_ptr)(int a, int b = 0);

add_func_ptr p = add; // 函数指针

int(&add_ref)(int, int = 0) = add; // 函数引用

int result = p(1); // 编译错误：参数中不能使用默认参数

int result2 = add_ref(1); // 编译错误：参数中不能使用默认参数

The solution is to add a default value to the parameter list of the function pointer or function reference:

typedef int(*add_func_ptr)(int a, int b /*= 0*/);

add_func_ptr p = add;

int(&add_ref)(int, int /*= 0*/) = add;

int result = p(1); // 成功调用add(1, 0)

int result2 = add_ref(1); // 成功调用add(1, 0)

In short, when we encounter an error that the default parameters cannot be used in the parameters, The code should be scrutinized to find out where the error is. If it is a problem with function overloading or function pointers and function references, default values ​​should be added to the parameter lists of all corresponding functions. For inconsistencies between function declaration and function definition, default values ​​should also be added to the parameters in the definition. As long as these simple processing methods are used, the error that the default parameters cannot be used in the parameters can be solved.

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