指针数组中的C ++错误

Question

I have a piece of code:

int CPUs = GetNumCPUs();
FILE *newFile[CPUs];

I got an error. It marks 'CPUs' on the second line and says: "expression must have a constant value".

I have tried to use const but it is not working.

Answer 1

You can't have a varible sized array in C++. Adding const to CPUs doesn't help, it only makes the variable read-only, but it's still not a compile time constant because it's initialized by a function at run-time.

The usual solution is to use a vector:

std::vector<FILE*> newFile(CPUs);

Answer 2

The value of GetNumCPUs() may or may not change every time you run the program - so it is not constant. If you want an array that has a variable amount of elements, try std::vector :

std::vector<FILE*> newFile(GetNumCPUs());

Answer 3

In your code, const doesn't mean "constant". In this context, it means the object is read-only — ie you can't modify the object. You're trying to create a variable length array, which isn't allowed in C++. Use std::vector , use new to allocate memory, or write a C99 program where VLAs like the one you're trying to make are allowed.

Answer 4

Using a const doesn't fix all of your problems in this scenario. The problem is that arrays must be initialized at compile time, so if you have a function return a variable ( GetNumCPUs() ) and assign it to a constant ( const int CPUs ), the variable isn't known at compile time but runtime, and the compiler can't allocate data space for the array.

Using an std::vector , however, allows for variable storage space.

std::vector<FILE*> newFile(CPUs);

This should work fine. Here's a couple tutorials:

http://www.codeguru.com/cpp/cpp/cpp_mfc/stl/article.php/c4027/C-Tutorial-A-Beginners-Guide-to-stdvector-Part-1.htm

http://www.dreamincode.net/forums/topic/33631-c-vector-tutorial/

Answer 5

在编译时不知道CPU。

Answer 6

You should dynamically create the array with new[]

int CPUs = GetNumCPUs();
FILE** newFile = new (FILE*)[CPUs];

After you're done with it, you're now responsible for deleting it as well:

delete[] newFile;