CODEWITHSUNDEEP
c++c++11
ronaldo
ronaldo

Reputation: 480

Cannot initialize a vector of const char*/string array with an initializer-list on declaration

Initially I started trying to initialize a vector of const char*[3] with an initilizer-list on declaration

vector<const char*[3]> v = { { "a", "b", "c" } };

And this gives the error

matrix must be initialized with a brace-enclosed initializer

I thought it might be due to the const char*, although it seemed odd, and changed it into strings

vector<string[3]> v = { { "a", "b", "c" } };

But the error persists. I tried several combinations of braces to no avail. Is it actually possible to initialize this structure on declaration with an initializer-list?

Upvotes: 5

Views: 2410

Answers (2)

Geezer
Geezer

Reputation: 5710

It fails to compile because std::vector requires its T to be CopyAssignable. No matter its RHS, this statement wouldn't not compile:

vector<const char*[3]> v = { { "a", "b", "c" } }; // Error

just as this wouldn't compile either:

std::vector<const char*[3]> v;
const char* charPtrArr[3] { "a", "b", "c" };
v.push_back(charPtrArr); // Error

This is just a particular case of the fact that C-style arrays are not assignable, demonstrated in code directly by using static_assert:

static_assert(std::is_copy_assignable<const char*[3]>()); // Error

or more generally I guess:

static_assert(std::is_copy_assignable<int[]>()); // Error

If you really wanted a std::vector of arrays of size 3 holding char pointers, then this is the error-free C++11 way to go about it:

vector<array<const char*, 3>> v = { { "a", "b", "c" }, { "d", "e", "f"} };

Upvotes: 7

Marek R
Marek R

Reputation: 37607

Problem is that C-style array can't be passed by copy or moved in function argument. As a result this code will not work

vector<const char*[3]> v;
const char* item[3] { "a", "b", "c" };
v.push_back(item); // will not compile

https://wandbox.org/permlink/LumvUzPnYWew7uMu
Basically this is the same problem, but initialization list is involved.

C++11 give you simple solution for your problem:

vector<array<const char*, 3>> v { { "a", "b", "c" }, { "d", "e", "f"} };

https://wandbox.org/permlink/IHNoSrH9BWV1IUoQ

Upvotes: 4

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