C++ Array size at run time without dynamic allocation is allowed?
声明：本页面是StackOverFlow热门问题的中英对照翻译，遵循CC BY-SA 4.0协议，如果您需要使用它，必须同样遵循CC BY-SA许可，注明原文地址和作者信息，同时你必须将它归于原作者（不是我）：StackOverFlow
原文地址: http://stackoverflow.com/questions/737240/
Warning: these are provided under cc-by-sa 4.0 license. You are free to use/share it, But you must attribute it to the original authors (not me):
StackOverFlow
Array size at run time without dynamic allocation is allowed?
提问by syaz
I've been using C++ for a few years, and today I saw some code, but how can this be perfectly legal?
I've been using C++ for a few years, and today I saw some code, but how can this be perfectly legal?
int main(int argc, char **argv)
{
size_t size;
cin >> size;
int array[size];
for(size_t i = 0; i < size; i++)
{
array[i] = i;
cout << i << endl;
}
return 0;
}
Compiled under GCC.
Compiled under GCC.
How can the size be determined at run-time without newor malloc?
How can the size be determined at run-time without newor malloc?
Just to double check, I've googled some and all similar codes to mine are claimed to give storage size error.
Just to double check, I've googled some and all similar codes to mine are claimed to give storage size error.
Even Deitel's C++ How To Program p. 261 states under Common Programming Error 4.5:
Even Deitel's C++ How To Program p. 261 states under Common Programming Error 4.5:
Only constants can be used to declare the size of automatic and static arrays.
Only constants can be used to declare the size of automatic and static arrays.
Enlight me.
Enlight me.
采纳答案by Mehrdad Afshari
This is valid in C99.
This is valid in C99.
C99 standard supports variable sized arrays on the stack. Probably your compiler has chosen to support this construct too.
C99 standard supports variable sized arrays on the stack. Probably your compiler has chosen to support this construct too.
Note that this is different from mallocand new. gccallocates the array on the stack, just like it does with int array[100]by just adjusting the stack pointer. No heap allocation is done. It's pretty much like _alloca.
Note that this is different from mallocand new. gccallocates the array on the stack, just like it does with int array[100]by just adjusting the stack pointer. No heap allocation is done. It's pretty much like _alloca.
回答by jpalecek
This is known as VLAs (variable length arrays). It is standard in c99, but gcc allows it in c++ code as an extension. If you want it to reject the code, try experimenting with -std=standard, -ansiand -pedanticoptions.
This is known as VLAs (variable length arrays). It is standard in c99, but gcc allows it in c++ code as an extension. If you want it to reject the code, try experimenting with -std=standard, -ansiand -pedanticoptions.
回答by ?zgür
It is validonly in C99. Next time you may try checking your code in a reliable compiler.
It is validonly in C99. Next time you may try checking your code in a reliable compiler.
回答by ?zgür
It is valid C99, it is not valid C++. This is one of not a few differences between the two languages.
It is valid C99, it is not valid C++. This is one of not a few differences between the two languages.
回答by Najeeb
You can give size to an array dynamically in if you are using Dev-Cpp compiler I have tried it and got no error but on visual c++ and visual studio compilers it is not possible. I think the reason is that dev-c++ assigns a positive number to the uninitialized int and when we give it a number it is replaced by the given one. but perhaps other compilers give null to uninitialized variables.
You can give size to an array dynamically in if you are using Dev-Cpp compiler I have tried it and got no error but on visual c++ and visual studio compilers it is not possible. I think the reason is that dev-c++ assigns a positive number to the uninitialized int and when we give it a number it is replaced by the given one. but perhaps other compilers give null to uninitialized variables.
回答by Dungeon
This Code runs in GNU GCC Compiler.
This Code runs in GNU GCC Compiler.
#include<bits/stdc++.h>
int main(int argc, char **argv)
{
size_t size;
std:: cin >> size;
int array[size];
for(size_t i = 0; i < size; i++)
{
array[i] = i;
std:: cout << i;
}
return 0;
}
回答by landerlyoung
I recently came across a scenario where a stack-allocated array is desired. (It's a wrapper around v8, needed an array of args on every method call).
I recently came across a scenario where a stack-allocated array is desired. (It's a wrapper around v8, needed an array of args on every method call).
An std::vector would do heap memory allocation, whose performance is not acceptable.
An std::vector would do heap memory allocation, whose performance is not acceptable.
Here is my solution, use template to allocation array of cases:
Here is my solution, use template to allocation array of cases:
template<size_t Argc>
static void call(...) {
v8::Local<v8::Value> v8Args[Argc];
// use v8Args
...
}
template<typename It>
static void callV8Function(size_t argc, It argvBegin, It argvEnd,) {
// C++ don't have dynamic stack allocation (like C99 does)
// try to avoid heap-allocation...
if (argc <= 4) {
return callV8FunctionOnStack<4>(...);
} else if (argc <= 8) {
return callV8FunctionOnStack<8>(...);
} else if (argc <= 16) {
return callV8FunctionOnStack<16>(...);
} else if (argc <= 32) {
return callV8FunctionOnStack< 32>(...);
} else {
std::vector<v8::Local<v8::Value>> v8Args(argc);
// fallback to vector
}
}
(And of course, I can just do with a 32-sized array, but which is not so elegant.)
(And of course, I can just do with a 32-sized array, but which is not so elegant.)
回答by Elkvis
Variable Length Arrays (VLAs) are supported in the C++14 standard, which has recently been accepted, and is awaiting publication.
Variable Length Arrays (VLAs) are supported in the C++14 standard, which has recently been accepted, and is awaiting publication.
