Programming_Micro_Note_20161007.md

#Programming Micro Note 2016100

Multiple definition

Source:
error LNK2005, already defined?
Q:
I have compilation errer with

//plus.h
int plus(int n){
    return n + 1;
}

//plus.cpp
#include "plus.h"
/*bla bla bla*/

//main.cpp
#include "plus.h"
int main(){
    plus(3);
}

A:
You violated One Definition Rule. Tracing like a compiler, you find that when compiling the plus.cpp translation unit, plus(int n) is defined twice since header file is copied by preprocessor. The solution is either to leave only declaration in header files and to put all implementation in cpp file or to announce the function as inline.

//plus.h
inline int plus(int n){
    return n + 1;
}

---

Dynamic allocation with size 0

Source:
C++ new int[0] — will it allocate memory?
Effective C++ 3/e - Item 51: Adhere to convention when writing new and delete

Q:
If a psycopath writes something like

int *pt = new int[0];

what happens?

A: It allocates an array with NO elements and return a legal pointer. Dereference the pointer is undefined behavior. You can do that but not legal. BTW, delete[] it is legal.

---

typename

Q:
I have a class looking like

template <typename T> 
class mw2dPoint : public mw2dGeometry<T> {
public:
    /*bla bla bla*/
    typedef typename misc::mwAutoPointer<mw2dPoint<T>> Ptr;
    /*bla bla bla*/
};

and when I write

template <typename T>
cadcam::mw2dPoint<T>::Ptr to_mwpoint(const contour_data<T>& contour){
    return new cadcam::mw2dPoint<T>(contour._pos[0], contour._pos[1]);
}

it doesn't compile!

A:
In compile time, the compiler doesn't know cadcam::mw2dPoint<T>::Ptr is a type (depending on T). Therefore, use

template <typename T>
typename cadcam::mw2dPoint<T>::Ptr to_mwpoint(const contour_data<T>& contour){
    return new cadcam::mw2dPoint<T>(contour._pos[0], contour._pos[1]);
}