A case of a lambda-expression bug

When solving the 9-50 exercises in C + + Primer 5th, the difference between lambda expression value capture and reference capture was encountered.

To modify the captured variable sum, Add. However, there is a difference between the forgotten value capture and the reference capture. The following code uses a value capture and fails to modify the value of the sum variable:

int Main () {    vector<string> v;     string s;     int 0 ;      while (Cin >> s)        V.push_back (s);    For_each (V.cbegin (), V.cend (),        [sum] (conststring &s) mutable {sum + =  Stoi (s); });     << sum;  // Output 0}

The right way:

Modifies the capture list of a lambda so that sum is expressed as &sum. The correct code after the modification is as follows:

 int   main () {vector  <string  > V;     string   S;  int  sum = 0  ;     while  (cin >> s) v.push_back (s); For_each (V.cbegin (), V.cend (), [ &sum] (const  string  &s) mutable {sum += Stoi (s);});  cout  << sum; //     output 1-10 cumulative and  return  0  ;}  

(End of full text)

A case of a lambda-expression bug