Why there is no compile error for the array definition with non-constant value?

Question

I thought I should get compile error for the following char array definition of the allData:

void MyClass::aMethod(const char* data, int size)
{
   int headerSize = 50;
   MyHeader header;
   //size is not constant and unknown at compile time
   char allData[size + headerSize]; //<<<<<==== should not allowed!! but not error??
   memcpy(allData, &header, headerSize);
   memcpy(allData + headerSize, data, size);
   ....
}

Why? It will give a run-time error?

Answered here: http://stackoverflow.com/questions/312116/c-array-size-dependent-on-function-parameter-causes-compile-errors?rq=1 — Jan Ladislav Dussek, Nov 04 '13 at 19:59
@chris I just tested it with -Werror -pedentic-errors -Wall with gcc and clang and the code compile fine. — Gabriel L., Nov 04 '13 at 20:01
You don't get an error, because ***it is not an error*** (provided you're using a C99 compiler, or a C++ compiler that supports it) — abelenky, Nov 04 '13 at 20:01
@abelenky it is an error for a standards compliant C++ compiler. — juanchopanza, Nov 04 '13 at 20:02
@GabrielL., Well, -pedentic-errors won't get you anywhere until you change the spelling. — chris, Nov 04 '13 at 20:10
@juanchopanza: I think I was pretty clear about saying it is not an error if your compiler supports it. Not all compilers are strictly 100% C++ compliant. — abelenky, Nov 04 '13 at 20:16
A diagnostic message is required for illegal code. A conforming compiler can choose to compile anyway. — jrok, Nov 04 '13 at 20:24

score 8 · Accepted Answer · edited Jun 20 '20 at 09:12

Both gcc and clang and possibly others although not visual C++, supports variable length arrays an extension even though it is a C99 feature not a C++ feature.

In both gcc and clang if you compile with -pedantic they will warn you that you are using extensions, for example gcc would produce a similar warning to this:

warning: ISO C++ forbids variable length array ‘allData’ [-Wvla]

and you can use -pedantic-errors to turn the warning into an error.

As far as I understand C++14 may support variable length arrays. The C99 draft standard section 6.7.5.2 Array declarators says:

[...] 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.

while the C++ draft standard requires a constant, the draft C++ standard in section 8.3.4 Arrays says:

In a declaration T D where D has the form

D1 [ constant-expressionopt] attribute-specifier-seqopt

[..] If the constant-expression (5.19) is present, it shall be a converted constant expression of type std::size_t and its value shall be greater than zero. [...]

score 2 · Answer 2 · answered Nov 04 '13 at 19:59

2

Some C++ compilers have an option that allows to use C VLA in C++.

answered Nov 04 '13 at 19:59

Vlad from Moscow

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Why there is no compile error for the array definition with non-constant value?

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