What does "new int[];" do?

Question

What does this line of code do?

new int[];

According to my compiler's disassembly (VC++ 2012), it does the same as:

new int[0];

But is it specified by the C++ standard? And is it a legal instruction?

Answer 1

The expression new int[] is not valid with C++11 (I have not checked C++14).

With a single [] the syntax requires an (implicitly convertible to) integral type expression between the brackets, denoting the desired array size.

Note that this size needs not be a constant: at the bottom level of abstraction this is how you allocate a dynamic array in C++.

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C++11 (via the N3290 final draft) §5.3.4/6:

” Every constant-expression in a noptr-new-declarator shall be an integral constant expression (5.19) and evaluate to a strictly positive value. The expression in a noptr-new-declarator shall be of integral type, unscoped enumeration type, or a class type for which a single non-explicit conversion function to integral or unscoped enumeration type exists (12.3)

expression is used for the first [] brackets. In subsequent [] brackets one must use a constant-expression (value known at compile time) because addressing of the outermost array dimension requires known size array items.

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Of course, instead of using new directly you will generally be better off using a std::vector (or maybe std::string).