用用户输入元素声明数组

Question

In C, when I declare an array of n elements a[n] , and n is input by the user, it shouldn't work, because the array space will change from static memory to dynamic memory.

But it works! Why ?

Has the standard been updated or something?

Answer 1

But it works! Why ?

Because C99 introduced variable length arrays . and you can declare dynamic arrays.

6.7.6.2 Array declarators:

If the size is an integer constant expression and the element type has a known constant size, the array type is not a variable length array type; otherwise, the array type is a variable length array type.

Answer 2

Variable length arrays ( VLA ) are a C99 feature, so the compiler you are using supports VLA or is supporting it outside of C99 mode as an extension, such as gcc and clang .

the C++ standard on the other hand does not support VLAs but many compilers including the ones I listed above support VLA in C++ as an extension.

We can see they were added in C99 by going to the draft C99 standard in the Foreword section it says:

[...]Major changes from the previous edition include:

and includes the following bullet:

— variable length arrays

Answer 3

The C99 standard allows for variable-length arrays, which is exactly the behaviour that your are describing.

Edit: C99 is not universally supported (at least by default). However, many compilers (such as GCC) have incorporated a subset of the C99 standard into their default compilation settings.

If you want to disable these features, you can do so in GCC by setting -ansi -pedantic in your compilation settings. This ensures that anything not supported in ANSI C will generate errors.