Difference between capture and passing an argument in lambda functions

Question

I understand the lambda function and the purpose of it in c++ 11. But i do not understand the difference between "Capturing the value" and "Passing an argument". For Instance..

#include <iostream> #include <functional> using namespace std;  int add(int a,int b){     return a+b; }  int main(int argc, char** argv){      function <int(int,int)> cppstyle;     cppstyle = add;      auto l = [] (function <int(int,int)> f,int a, int b) {return f(a,b);};      cout << l(cppstyle,10,30) <<"\n";    } 

The output of the code above is same as the code below..

#include <iostream> #include <functional> using namespace std;  int add(int a,int b){     return a+b; }  int main(int argc, char** argv){      function <int(int,int)> cppstyle;     cppstyle = add;      auto l = [cppstyle] (int a, int b) {return cppstyle(a,b);};      cout << l(10,30) <<"\n";     } 

Is "capturing a value" similar to "passing a value as an argument"? or capture has some special meaning?

Answer 1

The difference between a captured argument and a passing argument could be seen with an analogy. Consider the following function object:

struct Capture {   int &i;   int const j; public:   Capture(int &_i, int &_j) : i(_i), j(_j) {}   int operator()(int const a, int const b) {     i *= j;     return a * b;   } }; 

In function object class Capture there are two member variables i and j. There's also overloaded operator() which takes two input arguments. Now consider the following lambda:

int i, j; [&i, j](int const a, int const b) {   i *= j;   return a * b; }; 

The member variables of class Capture are in analogy with the lambda capture (i.e., [&i, j]), whereas input arguments of overloaded operator() a and b are in analogy with input arguments a and b of the lambda shown above.

That is, if you consider a lambda as a function object, its capture is the state of the function object (i.e., its member variables) whereas its input arguments would be the input arguments of the overloaded operator().