Thread: Unsure of how to properly use extern "C"

I have a child class

Code:

class IOLabel : public IOField
{
	private:
			int len;
			char format[11];
	public:
			IOLabel(int row, int col, int len); 
			IOLabel(const char *str, int row, int col);		
			~IOLabel(){};  //Deallocating non members?
			void display();
			int edit(){ 
				display();
				return 0;
			}
			IOLabel &setFormat(const char *format){ 
				strcpy(this->format,format);
				return *this;
			}
			IOLabel &operator=(const char *data){
				strncpy((char*)this->data,data,len); //Correct? Data null terminated
			}
			IOLabel &operator=(int number){ sprintf((char*)data,format,number);}
			IOLabel &operator=(double number){ sprintf((char*)data,format, number); } 
};

In my definition for one method I need to call a function written in C in the file cio.h. My professor said to use

Code:

extern "C" #include "cio.h"

but I am unsure how

Code:

void IOLabel::display(){
	extern "C"
	#include "cio.h"
	cio_display((char*)data,Row(),Col(),len);
}

Tried that but it didnt work

Code:

error C2598: linkage specification must be at global scope

Originally Posted by durban

extern goes before your definition, like so extern "C" void IOLabel::display()

Code:

scrio.obj : error LNK2019: unresolved external symbol "protected: int __thiscall IOField::Row(void)" (?Row@IOField@@IAEHXZ) referenced in function "public: virtual void __thiscall IOLabel::display(void)" (?display@IOLabel@@UAEXXZ)
scrio.obj : error LNK2019: unresolved external symbol "protected: int __thiscall IOField::Col(void)" (?Col@IOField@@IAEHXZ) referenced in function "public: virtual void __thiscall IOLabel::display(void)" (?display@IOLabel@@UAEXXZ)

Woops I forgot the definitions for row and col right

With another function in another class I get this error now

Code:

extern "C" void IOLineEdit::display(){

	#include "cio.h"
	cio_display((char*)data,Row(),Col(),flen);
	
}

Code:

error C3861: 'cio_display': identifier not found

When it obviously worked with the parent class's display method...

Code:

extern "C" void IOLabel::display(){

#include "cio.h"
	cio_display((char*)data,Row(),Col(),len);
}

Aha, I didnt know I had to declare it at the top of the program

Code:

extern "C"{ #include "cio.h"}

and then insert it at the begining of each function definition

Code:

extern "C" int whatever(int a){}