C Where to initialize variables in constructor

Question

Possible Duplicate    C   initialization lists       What are the pros cons of initializing variables at option 1 vs option 2   class MyClass   public      MyClass  float f  char a    private      float mFloat      char mCharacter      bool mBoolean      int mInteger      MyClass  MyClass  float f  char a     mFloat  f    mBoolean  true      option 1           option 2     mCharacter   a      mInteger   0      Edit  Why is option 2 so common

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There are many other reasons  You should always initialize all member variables in the initialization list if possible   http   www parashift com c  -faq-lite ctors html faq-10 6

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Option 1 allows you to use a place specified exactly for explicitly initializing member variables

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See Should my constructors use  initialization lists  or  assignment    Briefly  in your specific case  it does not change anything  But    for class struct members with constructors  it may be more efficient to use option 1  only option 1 allows you to initialize reference members  only option 1 allows you to initialize const members only option 1 allows you to initialize base classes using their constructor only option 2 allows you to initialize array or structs that do not have a constructor    My guess for why option 2 is more common is that option 1 is not well-known  neither are its advantages  Option 2 s syntax feels more natural to the new C   programmer

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In short  always prefer initialization lists when possible  2 reasons    If you do not mention a variable in a class s initialization list  the constructor will default initialize it before entering the body of the constructor you ve written  This means that option 2 will lead to each variable being written to twice  once for the default initialization and once for the assignment in the constructor body  Also  as mentioned by mwigdahl and avada in other answers  const members and reference members can only be initialized in an initialization list    Also note that variables are always initialized on the order they are declared in the class declaration  not in the order they are listed in an initialization list  with proper warnings enabled a compiler will warn you if a list is written out of order   Similarly  destructors will call member destructors in the opposite order  last to first in the class declaration  after the code in your class s destructor has executed

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Option 1 allows you to initialize const members  This cannot be done with option 2  as they are assigned to  not initialized    Why must const members be intialized in the constructor initializer rather than in its body

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Although it doesn t apply to this specific example  Option 1 allows you to initialize member variables of reference type  or const type  as pointed out below    Option 2 doesn t   In general  Option 1 is the more powerful approach

[c++] C++: Where to initialize variables in constructor

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