Pointer array and sizeof confusion

开发者_如何转开发

Why does the following code output 4?

char** pointer = new char*[1];
std::cout << sizeof(pointer) << "\n";

I have an array of pointers, but it should have length 1, shouldn't it?

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pointer is a pointer. It is the size of a pointer, which is 4 bytes on your system.

*pointer is also a pointer. sizeof(*pointer) will also be 4.

**pointer is a char. sizeof(**pointer) will be 1. Note that **pointer is a char because it is defined as char**. The size of the array new`ed nevers enters into this.

Note that sizeof is a compiler operator. It is rendered to a constant at compile time. Anything that could be changed at runtime (like the size of a new'ed array) cannnot be determined using sizeof.

Note 2: If you had defined that as:

char* array[1];
char** pointer = array;

Now pointer has essencially the same value as before, but now you can say:

 int  arraySize = sizeof(array); // size of total space of array 
 int  arrayLen = sizeof(array)/sizeof(array[0]); // number of element == 1 here.

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sizeof always returns a number of bytes.

Here, pointer is an ... err ... pointer and is 32 bits on 32 bits architectures, i.e. 4 bytes.

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When you call sizeof you're asking for how large it is in terms of bytes. A pointer is actually an integer that represents an address where the data you're pointing to is, and assuming that you're using a x32 operating system the size of an int is 4 bytes.

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pointer is of type char**, whic has size of 4

what you might want is char * pointer [1]

but to have the length of such array you need the following code

int len = sizeof(pointer)/sizeof(char*)

check this out:

int * pArr = new int[5];
int Arr[5];

sizeof(pArr); //==4 for 32 bit arch
sizeof(Arr); //==20 for 32 bit arch
sizeof(Arr)/sizeof(int); //==5 for any arch

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sizeof does not give you the size of dynamic arrays (whose size is only determined at run-time, and could be of different size during different executions).

sizeof is always evaluated at compile-time and it gives you the size of the type in question, in this case the type is char** - a pointer (to pointer).

It is your task to keep track of the size of dynamically allocated arrays (you know how much you requested in the first place). Since it is a burden, all the more reason to use containers and the string class, which keep track of the allocation size themselves.