Using const int as array size

Question

Why am I able to use a locally declared const int as the size of an array declaration but am not allowed to do the same with a const int passed as an argument?

For example, in the below code why do I get compiler errors only on line 2?

void f1(const int dim){
  int nums[dim];  // line 2: errors
}

void f2(){
  const int dim = 5;
  int nums[dim];  // ok
}

Answer 1

Array size should be known at compile time.

const int with local variables may do not work neither if the value is not known at compile time as:

void f2(){
  const int dim = bar();
  int nums[dim];  // error
}

In Both case, const int tells that the value doesn't change, not that is it known at compile time.

Answer 2

In f2 the constant dim is known at compile time, in f1 it's not.

And since C++ doesn't have variable-length arrays the dimensions of an array must be known at compile time.

The use of const for function arguments is more of a hint to the compiler that the function will not modify the argument, allowing the compiler to for example make optimizations based on that information.

Answer 3

Your understanding of the const keyword is wrong. const implies that within a certain contextual scope the variable will not change, not that the variable is determined at compile time.

Answer 4

Since c++11 though you can use constexpr which would do what you expect. The constexpr says compiler that apart the variable is const its value can be evaluated at compile time. See: http://en.cppreference.com/w/cpp/language/constexpr

Answer 5

1. Array size should be known at compile time.
2. const mean the value doesn't change.

So, this is about value of 'dim` at compile time.

In first case, it is unknown. compiler don't know the value of dim at compile time. It depends on the value passed to the function.

void f1(const int dim){
  int nums[dim];  // line 2: errors
}

In this case, dim value is known at compile time, so no issues.

void f2(){
  const int dim = 5;
  int nums[dim];  // ok
}

You can use a feature in c++ 11 for this.

define your function as constexpr .

constexpr int getDim(......)
{
    .......
    .......
    return dim;
}

then call getDim inside f1 .

void f1(){
  int dim = getDim(...); //// computed at compile time
  int nums[dim];
}

Note that, use of constexpr depend on your application. Further refer this .