vector< vector >: verify that all have equal sizes

Question

Is there an std/boost algorithm to verify that all vectors within a vector have the same sizes? And by extension, that a property of all elements is the same?

In the below examples, I use the hypothetical std::all_equal that I am looking for:

typedef std::vector<int> Line;
std::vector<Line> lines;
lines.push(Line(10));
lines.push(Line(11));

auto equalLengths = std::all_equal(lines.begin(), lines.end(), 
 [](const Line& x){ return x.size(); });

(And by extension:

std::vector<MyClass> vec;
auto equal = std::all_equal(std::begin(vec), std::end(vec),
 [](const MyClass& x) { return x.property(); });

)

Answer 1

How about

#include <algorithm> // for std::all_of

auto const required_size = lines.front().size();
std::all_of(begin(lines), end(lines),
    [required_size](const Line& x){ return x.size() == required_size; });

Won't work for empty lists, unfortunately and you have to get the required size into the predicate somehow.

Answer 2

I like @ComicSansMS's answer, but if you want a slightly less-clear approach that also works on empty vectors, you could use std::adjacent_find with a custom predicate:

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

int main() {
    std::vector<std::vector<int>> vv{{3, 1}, {4, 1}, {5, 9}};

    bool all_same_size = std::adjacent_find(
      vv.cbegin(), 
      vv.cend(), 
      [](const std::vector<int>& a, const std::vector<int>& b) {
        return a.size() != b.size();    // Look for two adjacent elements that
                                        // have different sizes
      }) == vv.cend();

    std::cout << "all same size: " << all_same_size << '\n';
}