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?

Answer 1

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. [...]

Answer 2

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