How to write a C wrapper for C++ lib for binary compatibility
- C wrapper can be developed to wrap lib(s) and dll(s) in older c++ versions or newer versions
Why need a C wrapper?
- Binary compatibility
- C++ version compatibility
- when having multiple projects 2019(C++ 14) or 2013(C++ 11) how debug them?
- use a C wrapper - they can be debugged or interconnected with any other c++ version
A C wrapper for a C++ binary is a C code that provides an interface to a C++ binary, allowing C programs to call C++ functions. This is useful when a C program needs to use a C++ library or binary, but cannot be modified to use C++ directly.
One way to create a C wrapper for a C++ binary is to write a C header file that declares the C++ functions and types that will be used by the C program. This header file can then be included in the C program, allowing it to call the C++ functions. However, since C++ and C have different naming conventions, a C++ binary may not be compatible with C, which can lead to linker errors.
Another way to create a C wrapper for a C++ binary is to use the "extern C" keyword. This keyword tells the C++ compiler to use C-style name mangling for the functions and types declared within the "extern C" block, making them compatible with C.
For example, the following is an example of a C wrapper for a C++ function:
extern "C" {
int AddNumbers(int a, int b) {
return a + b;
}
}
In this example, the C++ function "AddNumbers" is declared with the "extern C" keyword, which tells the compiler to use C-style name mangling for this function. As a result, the C program can call this function just like a normal C function.
In the case of C++ classes, a C wrapper can be created by creating a C struct with the same layout and providing C functions to interact with the C++ class, this way the C program can access the C++ class's functionality without knowing it's a C++ class.
Using a C wrapper for a C++ binary can make it easier to integrate C++ libraries and binaries into C programs, but it can also introduce additional complexity and overhead. It's important to consider the trade-offs and the specific requirements of your project before deciding to use a C wrapper for a C++ binary.
Example code
#ifndef EXAMPLE_HPP
#define EXAMPLE_HPP
class Example {
public:
Example();
int add(int a, int b);
int sub(int a, int b);
void print(const char* message);
private:
int x;
};
#endif
C++ implementation file (example.cpp):
#include "example.hpp"
#include <iostream>
Example::Example() : x(0) {}
int Example::add(int a, int b) {
return a + b;
}
int Example::sub(int a, int b) {
return a - b;
}
void Example::print(const char* message) {
std::cout << message << std::endl;
}
C wrapper file (example_wrap.c):
#include "example.hpp"
extern "C" {
Example* Example_new() {
return new Example();
}
int Example_add(Example* obj, int a, int b) {
return obj->add(a, b);
}
int Example_sub(Example* obj, int a, int b) {
return obj->sub(a, b);
}
void Example_print(Example* obj, const char* message) {
obj->print(message);
}
void Example_delete(Example* obj) {
delete obj;
}
}
In this example, the C++ class "Example" has three methods: add, sub and print. The C wrapper provides C functions that are equivalents of the C++ methods: Example_new, Example_add, Example_sub, Example_print and Example_delete.
The C wrapper functions are declared with the "extern C" keyword, this tells the C++ compiler to use C-style name mangling for these functions. This makes them compatible with C
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