c++ - error C3499: a lambda that has been specified to have a void return type cannot return a value -

i using following lambda , getting error. can't figure out why compiler doesn't doing.

std::string captchaword(6, 0); std::generate(captchaword.begin(), captchaword.end(), []() {     unsigned int num = randomizer('z' - 'a' + 1 + '9' - '0' + 1);     char ch = num + 'a';     if (num >= 'z' - 'a' + 1)     {         ch += '0' - 'z' - 1;     }     return ch; }); 

by way, randomizer function following signature:

unsigned int randomizer(unsigned int); 

this error message get:

error c3499: lambda has been specified have void return type cannot return value.

you have specify return type:

[]() -> char { // code; } 

the automatic deduction works if whole lambda consists of single return statement only (in c++11), otherwise, need specify type. see documentation on lambda on cppreference. in c++14, rules allow other statements before return.