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Home » Tip Bank » C++
Language: C++
Expertise: All
May 21, 1999

Relocating Declarations Can Enhance Performance

On some occasions, the performance boost that can result from moving declarations is quite considerable. Consider this example:
 
  void use()  {
    string  s1;
    if (condition == false){
      return; //s1 was not needed
    }   

    s1 = "hello";
    // s1 used only here
    return; 
  }
The local object s1 is unconditionally constructed and destroyed in use(), even if it is not used at all. As you can see, when the condition in the if-statement is false, the unnecessary construction and destruction of s1 are still unavoidable. Nevertheless, with a little help from the programmer, the unnecessary construction and destruction of s1 can be eliminated. All that is required is to relocate the declaration of s1 to the point where it is actually used:
 
  void use(){
    if (condition == false){
      return; 
    }   
    string  s1; //moved from the block's beginning
    //use s1 here
    return; 
  }
Consequently, the object s1 is constructed only when it is really needed. When it is not needed, s1 is neither constructed nor destroyed. Thus, simply by moving the declaration of s1, you eliminate two unnecessary member function calls.
Danny Kalev
 
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