Explicit Return Type of Lambda

Question

When I try and compile this code  VS2010  I am getting the following error  error C3499  a lambda that has been specified to have a void return type cannot return a value  void DataFile  removeComments       string  const iterator start  end    boost  regex expression     s        boost  match results lt std  string  const iterator gt  what    boost  match flag type flags   boost  match default       Look for lines that either start with a hash          or have nothing but white-space preceeding the hash symbol   remove if rawLines begin    rawLines end      amp expression   amp start   amp end   amp what   amp flags  const string amp  line          start   line begin        end   line end        bool temp   boost  regex search start  end  what  expression  flags       return temp            How did I specify that the lambda has a  void  return type  More-over  how do I specify that the lambda has  bool  return type   UPDATE  The following compiles  Can someone please tell me why that compiles and the other does not   void DataFile  removeComments       boost  regex expression      s          boost  match results lt std  string  const iterator gt  what    boost  match flag type flags   boost  match default       Look for lines that either start with a hash          or have nothing but white-space preceeding the hash symbol   rawLines erase remove if rawLines begin    rawLines end      amp expression   amp what   amp flags  const string amp  line      return boost  regex search line begin    line end    what  expression  flags

User · Accepted Answer

You can explicitly specify the return type of a lambda by using -> Type after the arguments list:

[]() -> Type { }

However, if a lambda has one statement and that statement is a return statement (and it returns an expression), the compiler can deduce the return type from the type of that one returned expression. You have multiple statements in your lambda, so it doesn't deduce the type.

User · Answer

The return type of a lambda  in C  11  can be deduced  but only when there is exactly one statement  and that statement is a return statement that returns an expression  an initializer list is not an expression  for example   If you have a multi-statement lambda  then the return type is assumed to be void   Therefore  you should do this     remove if rawLines begin    rawLines end      amp expression   amp start   amp end   amp what   amp flags  const string amp  line  - gt  bool         start   line begin        end   line end        bool temp   boost  regex search start  end  what  expression  flags       return temp         But really  your second expression is a lot more readable

User · Answer

You can have more than one statement when still return        - gt  your type  return           your statement          even more statement   just add comma          return value      http   www cplusplus com doc tutorial operators  comma

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