C++ Array Error: "variable-sized object may not be initialized"

Question

Any idea why this isn't working in C++? I feel like it's really simple, but I can't figure it out:

int main() {
  int arraySize = 5;
  char testArray[arraySize] = {'a', 'b', 'b', 'c', 'd'};
}

It works fine when I hard code 5 as the array size, but it doesn't like it when I use a variable name instead. Ultimately I'm trying to figure out how to write a function that deletes duplicate characters from the array, but I can't even get the array initialized.

Consider using [`std::vector`](https://en.cppreference.com/w/cpp/container/vector). — MikeCAT, Jun 29 '21 at 13:09
Does this answer your question? [Why aren't variable-length arrays part of the C++ standard?](https://stackoverflow.com/questions/1887097/why-arent-variable-length-arrays-part-of-the-c-standard) — 463035818_is_not_an_ai, Jun 29 '21 at 13:10
*Any idea why this isn't working in C++?* -- Because it isn't C++. — PaulMcKenzie, Jun 29 '21 at 13:12
`const int arraySize = 5;` would make the code ok, but you really want a `std::vector` or a `std::string` — 463035818_is_not_an_ai, Jun 29 '21 at 13:14
char testArray[] = ['a', 'b', 'b', 'c', 'd'};` is all you need. The compiler will infer the size of the array from the number of elements in the initializer list. — Pete Becker, Jun 29 '21 at 13:37

Marek R · Answer 1 · 2021-06-29T13:25:52.060

By default compilers are expecting mix of C++ and C code. So VLA is enabled as an extension. It is possible to enfore pure C++ then this code will fail to compile.

Note that msvc supports old C standard where VLA was not available yet, so in many multi-platform projects compilers are configured not to use VLA.

Now this warning:

variable-sized object may not be initialized

Basically says, you have requested VLA and you are initializing it with 5 elements. It may happen that arraySize variable can be changed during runtime to value less then 5 and this can corrupt your initialization code. That is why this warning is reported. Some initialization may be uncompleted.

Now your question was closed as duplicate indicating that C++ standard do not support VLA. I reopened it since question was about warning what is not covered by duplicate.

VLA were only required in C99 and are an optional feature since C11... — Serge Ballesta, Jun 29 '21 at 13:30

C++ Array Error: "variable-sized object may not be initialized"

1 Answers1