Exclusive Prefix Sum

#include <vector>
#include <iostream>
#include <algorithm>
#include <iterator>

template<typename T>
struct Scan {

    T value;
    
    Scan(T init) : value(init){
	
    }

    T operator()(const T &element){
	T prevValue = value;
	value = value + element;
	return prevValue;

    }
    
};


int main(){


    std::vector<int> input{{1,2,3,4,5}};

    std::transform(input.begin(), input.end(), std::ostream_iterator<int>(std::cout, " "), Scan<int>(0));
    
    return 0;
}

std::transform does not guarantee in-order application of unary_op or binary_op. To apply a function to a sequence in-order or to apply a function that modifies the elements of a sequence, use std::for_each

source

A better and shorter solution with for_each and lambdas:

    int value = 0;
    std::for_each(input.begin(), input.end(), [&value](int &n){
        int prevValue = value;
        value += n;
        n = prevValue;
    });