(C++) operator--

 

operator-- (also called the decrement operator) decreases an integer's value by one. To increase an integer's value by one, use operator++

 

operator-- has a pre-fix and post-fix form, which are '--i' and 'i--' respectively. The prefix form returns the value of i its new value, the postfix form returns the value of i before its decrement. Prefer '--i' over 'i--' [1].

 

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#include <cassert> int main() {   int i = 0;   --i; // preferred [1]   assert(i==1);   i--; // not preferred [1]   assert(i==2); }

 

There are four ways to decrement a value by 1, that make use of different operators and some of temporary copies.

 

+--------------------------------------+--------------------------------------+ | Code | Temporary copy yes/no | +--------------------------------------+--------------------------------------+ | i = i - 1; | yes | +--------------------------------------+--------------------------------------+ | i -= 1; | no | +--------------------------------------+--------------------------------------+ | --i; | yes | +--------------------------------------+--------------------------------------+ | i--; | no | +--------------------------------------+--------------------------------------+

 

 

 

 

 

Overloading operator--

 

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struct MyInt {   //Prefix   MyInt& operator--()   {     --mX;         //Decrement     return *this; //Return class reference   }   //Postfix   MyInt operator--(int)   {     MyInt old(*this); //Copy     --(*this);        //Decrement original using prefix     return old;       //Return old copy   }   int mX; }; int main() {   MyInt m;   --m;   m--; }

 

 

 

 

 

References

 

1. Bjarne Stroustrup. The C++ Programming Language (3rd edition). 1997. ISBN: 0-201-88954-4. Item 19.5.7: 'Prefer ++p to p++'