Is there any way of creating a static array in c++ at run time. What i want is really simple like just want to get input number from user and create a static array of size of input number at run time. No new operator is required no pointer are required just static array?
6 Answers
No. static variable is allocated before the program code is actually running (i.e.: before your main is called). What you need is a dynamic (aka created at run time) array. If you want to avoid new you can create it on stack (by passing parameter to a function that will create it and working on it within that function), but that's not the same as static. You can also use template containers that will do the allocation and resizing for you (like std::vector, mentioned in other answers)
edit
It seems to bother some people that I didn't mention the matter of initializing static objects. Although not directly relevant to the question - worth to know that static member variables or static variables within a scope can be initialized at run time, but the space for them is reserved prior to the main, so the size of the variable cannot be changed.
5 Comments
Lightness Races in Orbit
I'm not sure that this is the meaning of "static" the OP is talking about.
Ben Voigt
The storage is created before the program starts... when the constructor is run depends on whether it's a global, class static member, or local static.
Ben Voigt
@littleadv: Array construction includes running a constructor for each element.
Ben Voigt
@littleadv: Reading your answer, I would think that I couldn't pass arguments determined at run-time to the constructor of a static local variable, but that's not the case.
Ben Voigt
@littleadv: It's the first complete sentence: "
static variable is created before the program code is actually running". "created" means the constructor runs. You probably are looking for "allocated" or "storage is reserved".If you mean
unsigned size;
std::cin >> size;
int arr[size];
Then: No. C99 has a feature called Variable-Length-Arrays, but the C++03 (and '0x) standard have no notion of this kind of feature.
2 Comments
Marlon
I think you mean
std::cin ;)
Aaron McDaid
In my answer I've described how
alloca can be used to allocate memory on the stack in C++, this is equivalent to C99's variable length arrays.Use alloca to allocate space on the stack, just like a static array or like a C99 variable length array.
#include <iostream>
#include <alloca.h>
int main() {
unsigned sz;
std :: cin >> sz;
int * p = static_cast<int*>(alloca(sz * sizeof(int)));
// do stuff with p, do not attempt to free() it
}
I've only ever used it with C, but it works well. Read about it first though. It probably isn't very portable.
Comments
No.
By definition, you need to dynamically allocate anything whose size is not known until runtime. That's why new and friends exist.
answered Jun 12, 2011 at 23:05
Lightness Races in Orbit
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Comments
What does it mean to you that an array is static? What advantages do you think it gives you?
Actually, any static variable (including arrays) has its storage reserved at the beginning of the program. For this reason, its size has to be known before the program starts running, so it can't depend on user input in any way.
There are a number of ways to make dynamic non-static arrays, however, so let us know what you're trying to do.
2 Comments
Lightness Races in Orbit
It wasn't me, but I'm not convinced that this strictly answers the question. Isn't it more of a comment?
oh whatever
Not a answer. The question can be answered by "yes" or "no", not by "I don't know what you mean". If you want - you can put it as a comment. What I understand from your answer is that you don't know, so why bother?
It sounds like what you want is an array whose size is run-time defined but whose life time is defined by static storage duration (ie the length of the program).
You can not use the basic array built into the language. The size of these objects are defined at compile time and can not be modified at run-time. But there are a couple of alternatives.
The basic std::vector is probably what you want:
std::vector<int> data;
int main()
{
int size;
std::cout << "Input the size of the array\n";
std::cin >> size;
// This line sets the size of the container
// vector is basically a C++ wrapper around a dynamically sized array
data.resize(size);
// Now we can safely read values into the array (like) container.
for(int loop =0;loop < size;++loop)
{
std::cout << "Input Value " << loop << "\n";
std::cin >> data[loop];
}
}
static?staticit may help clarify the situation.