usage of const in c++

Question

I am new to C++.I was going through a C++ book and it says

const int i[] = { 1, 2, 3, 4 };
float f[i[3]]; // Illegal

It says the declaration of the float variable is invalid during compilation.Why is that?

Suppose if we use

int i = 3;
float f[i];

It works.

What is the problem with the first situation?

Thanks.

Answer 1

So the first is illegal because an array must have a compile-time known bound, and i[3], while strictly speaking known at compile time, does not fulfill the criteria the language sets for "compile-time known".

The second is also illegal for the same reason.

Both cases, however, will generally be accepted by GCC because it supports C99-style runtime-sized arrays as an extension in C++. Pass the -pedantic flag to GCC to make it complain.

Edit: The C++ standard term is "integral constant expression", and things qualifying as such are described in detail in section 5.19 of the standard. The exact rules are non-trivial and C++11 has a much wider range of things that qualify due to constexpr, but in C++98, the list of legal things is, roughly:

• integer literals
• simple expressions involving only constants
• non-type template parameters of integral type
• variables of integral type declared as const and initialized with a constant expression

Answer 2

Your second example doesn't work and it shouldn't work. i must be constant. This works

const int i = 3;
float f[i];