C++ 如何使用 lambda 表达式来捕获之前的迭代值？

Question

I'm using this simple example of creating a simple vector with its element in strictly increasing, random step manner. I have an implementation using function objects:

struct IncrementGenerator
{
    int m;
    IncrementGenerator() : m(0) {}
    int operator()()
    {
        m += 1 + (rand() % 5);     //random Increment between [1,5]
        return m;
    }
};

vector<int> myVector(10);
generate(myVector.begin()+1, myVector.end(), IncrementGenerator());

//example of the output: 0, 4, 6, 9, 10, 14, 15, 17, 20, 25,

I've just learnt lambda expression today and would like to do the same implementation in much short code. I'm not sure if I can capture the previous iteration in the lambda expression and add on a random number onto it:

// my idea is such, but not really sure what to write in the expression so i put some question mark.
generate(myVector.begin()+1, myVector.end(),[?]()->int{return ? + (1 + (rand()%5)) });

Answer 1

The lambda equivalent to the class above is this:

auto increment_generator = [m = 0]() mutable
{
    m += 1 + (rand() % 5);     //random Increment between [1,5]
    return m;
};

Note the mutable modifier which indicates that the captured m is not constant. By default, variables captured by lambdas are const .