c++11 - What does "[&]" mean in C++? -

i reading source code , contains following lines.

auto updateslack = [&](const char slack_type, const double s) {     if (slack_type == 'e') {         wns.early_slack = min(s, wns.early_slack);         tns.early_slack += s;     }     else if (slack_type == 'l') {         wns.late_slack = min(s, wns.late_slack);         tns.late_slack += s;     } }; 

i confused "[&]" symbol after "=" operator. can tell suggests?

this statement uses lambda expression.

auto updateslack = [&](const char slack_type, const double s) {     if (slack_type == 'e') {         wns.early_slack = min(s, wns.early_slack);         tns.early_slack += s;     }     else if (slack_type == 'l') {         wns.late_slack = min(s, wns.late_slack);         tns.late_slack += s;     } }; 

the compiler 2 things. first 1 compiler defines unnamed class contain function call operator corresponds body of lambda expression.

the second 1 compiler creates object of class , assign variable updateslack.

you can imagine following way

struct   {     auto &r1 = wns;     auto &r2 = tns;     void operator ()( const char slack_type, const double s ) const    {         if (slack_type == 'e') {             wns.early_slack = min(s, wns.early_slack);             tns.early_slack += s;         }         else if (slack_type == 'l') {             wns.late_slack = min(s, wns.late_slack);             tns.late_slack += s;         }     } } updateslack; 

then can call function call operator of object somewhere in program functional object needed like

updateslack( 'l', 10.10 );