C++ error: Pointer to non-object cannot be converted to other pointer type. What should I do?

In recent years, C has been one of the programming languages ​​favored by developers. As a high-level programming language, C has many powerful features, such as type casting. Casting is a very important concept in C and is necessary in many situations. However, errors sometimes occur during casts, especially when converting pointer types to other types. This article will introduce pointer type conversion errors in C and provide corresponding solutions.

In C, pointers are a very key concept. A pointer is a variable whose value is the address of another variable. When working with pointers, it often happens that one pointer type is converted to another type. In some cases, pointer type conversion is necessary, such as when converting a pointer to a base class to a pointer to a derived class. However, this conversion may cause errors or exceptions. One of the situations where C pointer type conversion errors occur is when a pointer to a non-object is converted to another pointer type.

A pointer to a non-object refers to a pointer variable, and the value it points to is not a valid object. Common causes are the use of uninitialized pointer variables or deleted objects. In this case, converting the pointer type to another type may cause the system to crash. For example:

int* p = nullptr;
double* pd = static_cast<double*>(p);

In the above code, the pointer variable p is initialized to nullptr, which points to a null address, so p does not point to a valid object. Next, we try to cast the pointer variable p to a double type pointer pd, but such a cast is unsafe because the memory space pointed to by the pointer p does not contain any useful information or data.

To solve this problem, before converting a pointer to another type, first check whether the object pointed to by the pointer is a valid object. If the pointer is nullptr or the object pointed to has been deleted, then the pointer should be avoided or reinitialized so that it points to a valid object. For example:

int* p = nullptr;
if (p != nullptr) {
    double* pd = static_cast<double*>(p);
}

In the above code, we first check whether the pointer variable p is nullptr. If p is nullptr, then the pointer type conversion operation will not be performed.

Another way to solve the problem of pointer to non-object conversion error is to use the dynamic_cast keyword. dynamic_cast is a runtime type identification (RTTI) operation in C that is used to convert a pointer to a base class into a pointer to a derived class. Regarding the use of dynamic_cast operation, you can refer to the following example:

#include <iostream>
using namespace std;

class Base {
public:
    virtual void Display() {
        cout << "This is Base class" << endl;
    }
};

class Derived : public Base {
public:
    virtual void Display() {
        cout << "This is Derived class" << endl;
    }
};

int main() {
    Base* p = new Derived();
    Derived* pd = dynamic_cast<Derived*>(p);
    if (pd != nullptr) {
        pd->Display();
    }
    delete p;
    return 0;
}

In this example, we define a base class Base and a derived class Derived. First we define a pointer p pointing to the base class and initialize it to point to an object of the Derived class. Then we use dynamic_cast to convert the pointer p to the base class into the pointer pd to the derived class. If the pointer type conversion is successful, then we can call the Display method of pd to output the "This is Derived class" information.

In short, casting in C is a very important operation that may have a huge impact on the performance and correctness of the program. Especially when converting pointer types to other types, you need to pay special attention because this operation may cause an exception or crash in the program. For the case where an error occurs when a pointer to a non-object is converted, we can effectively solve the problem by checking whether the object pointed to by the pointer is a valid object or using the dynamic_cast keyword.

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