- A+

This code:

`#include <vector> #include <string> #include <iostream> class MyClass { public: MyClass(const std::vector<std::vector<std::string>> & v) { std::cout << "Vector of string vectors size: " << v.size() << "/n"; for (size_t i = 0; i < v.size(); i++) std::cout << "Vector #" << i << " has size " << v[i].size() << "/n"; } }; int main() { MyClass({ { "a" } }); // <--- ok MyClass({ { "a", "b" } }); // <--- PROBLEM MyClass({ { std::string("a"), "b" } }); // <--- ok MyClass({ { "a", "b", "c" } }); // <--- ok MyClass({ { "a" },{ "c" } }); // <--- ok MyClass({ { "a", "b" },{ "c", "d" } }); // <--- ok } `

outputs this (Visual Studio 2017):

`Vector of string vectors size: 1 Vector #0 has size 1 Vector of string vectors size: 4 Vector #0 has size 97 Vector #1 has size 0 Vector #2 has size 0 Vector #3 has size 0 Vector of string vectors size: 1 Vector #0 has size 2 Vector of string vectors size: 1 Vector #0 has size 3 Vector of string vectors size: 2 Vector #0 has size 1 Vector #1 has size 1 Vector of string vectors size: 2 Vector #0 has size 2 Vector #1 has size 2 `

So, it works OK in all cases except in the case where we have a vector of one vector, containing two strings. It also works in the above case if we explicitly construct std::string from one of the string literals. If both are just plain string literals, the compiler seems to get "confused" and constructs a vector of 4 items, the first of which contains 97 strings. Note that 97 is the character code of "a".

I guess my question is, should the compiler interpret this problematic construction as I'd expect, or is this bad code to initialize a nested list like this?

The inner vector in `MyClass({ { "a", "b" } })`

is creating using range constructor:

`template <class InputIterator> vector (InputIterator first, InputIterator last, const allocator_type& alloc = allocator_type()); `

This happens because `{ "a", "b" }`

is interpreted not as `std::initializer_list<std::string>`

but as a pair of raw pointers.