How to Properly Allocate Memory for C Structures with String Members?

Using C Strings in Structures Allocated with malloc

When allocating memory for C structures using malloc, it is important to consider the presence of data members that require non-trivial constructors, such as std::string.

Problem and Code Example

Consider the following code that attempts to allocate memory for a structure containing a std::string member:

#include <iostream>
#include <string>
#include <cstdlib>

struct example {
    std::string data;
};

int main() {
    example *ex = (example *)malloc(sizeof(*ex));
    ex->data = "hello world";
    std::cout << ex->data << std::endl;
}

This code will likely result in a segmentation fault. The issue arises because malloc allocates raw memory that is not automatically initialized, leaving the std::string member in an uninitialized state.

Solution: Using C Memory Allocation Operators

The proper way to allocate memory for C objects is to use C memory allocation operators, such as new and delete. These operators automatically call the constructor and destructor for the allocated object, ensuring proper initialization and cleanup.

For the provided code, the solution is to replace malloc with new:

example *ex = new example;

Alternative: Placement New

If you need to allocate memory with malloc but still want to use C constructors, you can use the placement new operator:

void *ex_raw = malloc(sizeof(example));
example *ex = new(ex_raw) example;

Placement new allows you to construct an object in a specific location in memory. However, this approach is not recommended unless necessary for specific reasons.

Conclusion

When dealing with C structures that contain non-trivial data members, it is essential to use proper memory allocation techniques with new and delete to avoid undefined behavior or errors.

The above is the detailed content of How to Properly Allocate Memory for C Structures with String Members?. For more information, please follow other related articles on the PHP Chinese website!