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Is there any way to initialize an array with a non-const integer, or to make the existing variable constant in order to make it a valid argument?

bool f( const char s[], const int n )
{
    char c[n]; // error: expression must have a constant value
}
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  • either as new char[n] or std::vector Commented Jun 24, 2015 at 13:02
  • There is no standard way for this, some compilers do support it by extension (stackoverflow.com/a/5377465/3747990). A vector would be a better alternative. Commented Jun 24, 2015 at 13:04
  • It may work if it's a inline function and it's called with a int literal. Commented Jun 24, 2015 at 13:10

3 Answers 3

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No, not in the general case. Use vector<char> c(n) instead.

Simplified, almost correct explanation: if you don't know what n is at compile time, neither does the compiler. So it cannot put aside memory for the array. This is why vector exists.

You can always use &c[0] to get the pointer to char if you need it elsewhere.

But it is possible in C99, apparently. Thanks to @Matt McNabb for pointing this out. If you can wait a few years you might be able to compile it in C++, too. In the meanwhile, use vector.

If you insist to have an "array" in C++, you would have to do something like:

char* c = new char[n];

If your program does not run forever, or do this too often, you can even just leave it as it is and not bother deleting. Tools like Valgrind might complain though.

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6 Comments

... or to allocate memory on heap?
@Kris Correct. Removed the misleading comment.
The "Explanation" isn't right: C does allow runtime-sized arrays, so the compiler can clearly figure it out. The reason C++ doesn't is because it has repercussions throughout the whole type system, and nobody has been able to figure out how to cope with this in C++ yet. (think templates...)
@MattMcNabb Is the explanation in the link I provided correct? (Thank you for the pointer, btw, this is something I did not know).
I have tried using size_t initialized with n, but I still get the same type error.
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Depending on the source of n, the answer is probably no. In case n can be a constexpr, then the answer is yes.

See this SO post for more information about constexpr: When should you use constexpr capability in C++11?

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n is located on a stack. In this case a compiler needs to know the size of n at compile time. You can dynamically allocate memory by operator new, or use std::vector.

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