Parameter passing to std::accumulate

My reference is to the example provided under the following:

std::accumulate

I am trying to constrast the following usage of std::next in the example,

std::string s = 
  std::accumulate(
    std::next(v.begin()), 
    v.end(),
    std::to_string(v[0]), // start with first element
    dash_fold
  );

with the usage example provided for std::next

Given that in the referred to example for std::accumulate usage, the following is the signature of dash_fold:

auto dash_fold = [](std::string a, int b){...}

I am assuming that the construct std::next(v.begin()), v.end(), std::to_string(v[0]...
above should produce

int, string

which is reversing the order of parameters that dash_fold expects; is this understanding correct? Or am I missing something?

TIA

>Solution :

The referenced manual is very detailed.

constexpr T accumulate( InputIt first, InputIt last, T init,
                        BinaryOperation op );

T in your case is the type of std::to_string(v[0]), is std::string.

op:

Ret fun(const Type1 &a, const Type2 &b);

The type Type1 must be such that an object of type T can be implicitly converted to Type1.

Type1 must be such that an object of type std::string can be implicitly converted to Type1. If the first parameter type is int, then std::string cannot be implicitly converted to int – fail. If the first parameter type is std::string, then it works well.