Has C++ Always Allowed Using A Variable for Array

问题:

This question already has an answer here:

• In C++ books, array bound must be constant expression, but why the following code works? 2 answers

For some reason, in the past, I recall not being able to do something like:

int arraySize;
cin >> arraySize;

int array[arraySize];

But recently, I tried this again and its not causing any issues. I could've sworn before this was something that threw an error in my compiler (macOS Sierra, Xcode 8.1). Was anything in the language updated to allow this? - I could be entirely remembering incorrectly and this wasn't an issue before, but I'm not sure. I thought array sizes had to be defined during compilation and the user couldn't pick that (which is where you would implement a dynamic array).

回答1:

The C++ Standard does not support variable length arrays though some compilers can have their own language extensions that allow to use VLAs in a C++ program.

Thus this code snippet

int arraySize;
cin >> arraySize;

int array[arraySize];

is not C++ compliant.

Use instead the standard C++ class std::vector.

As for C then according to the C Standard implementations may conditionally support VLAs.

You can check whether an implementation supports VLAs. From the C Standard (6.10.8.3 Conditional feature macros)

1 The following macro names are conditionally defined by the implementation:

__STDC_NO_VLA__

The integer constant 1, intended to indicate that the implementation does not support variable length arrays or variably modified types.

回答2:

Was anything in the language updated to allow this

No. Variable length arrays (aka. VLAs) are a compiler specific extension.

The c++ standard never allowed this (unlike the c99 standard does so contrarily).