sizeof works unusual at c++ after array creation

Question

Possible Duplicate:
How to find the sizeof(a pointer pointing to an array)

I am creating an array by using following code

float *A;
A = (float *) malloc(100*sizeof(float));
float *B;
B = (float *) malloc(100*sizeof(float));

but after these when I type an print the size of the A and B by the following, I get 2 as a result as I expect to see 100.

sizeof(A)/sizeof(float)

Your expectation, that the size of a pointer should change if you change what it's pointing to, is unreasonable. (Now, if this was an array passed to a function, you'd have a point.) — David Schwartz, Jan 27 '13 at 00:40

score 11 · Answer 1 · answered Jan 27 '13 at 00:27

11

Your expectation is wrong. A is a float*, so its size will be sizeof(float*), regardless of how you actually allocate it.

If you had a static array - i.e. float A[100], then this would work.

Since this is C++, use std::array or std::vector.

Worst case, use new[]. Definitely don't use malloc.

answered Jan 27 '13 at 00:27

Luchian Grigore

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so no way to get the size ? – erogol Jan 27 '13 at 00:29
8

@Erogol there would be if you took good advice and stop writing C code and labeling it C++. Use `vector` or `array`. – Luchian Grigore Jan 27 '13 at 00:30
I think best is to learn C and stop scripting with toy languages... – erogol Jan 27 '13 at 00:32
@Erogol, just store the size in a variable or use higher-level C++ constructs – nameless Jan 27 '13 at 00:33
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@Erogol so, wait, do you want to learn C or C++? You do know they're different, right? – Luchian Grigore Jan 27 '13 at 00:34
@Erogol, if you want to learn C, then learn it. But why do you tag your question with `C++` tag then? Don't you know that C and C++ are different languages? – nameless Jan 27 '13 at 00:36
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@Erogol If you tag your question `C++`, expect C++ answers. – Etienne de Martel Jan 27 '13 at 00:39
Ohh man are these different? I should have realized that before as their difference on the names? You enlightened on me ? Regards for all your bright minds? – erogol Jan 27 '13 at 00:45
@EtiennedeMartel I would say f*ck off but it is not appropriate – erogol Jan 27 '13 at 01:48
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@Erogol Sorry, you can't have an account here if you're under 13. – Cat Plus Plus Jan 27 '13 at 02:05
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@Erogol Stop throwing tantrums like a spoiled stubborn child. You got your answer already. Annoy and insult more people who try to help you even though they don't have to, see how well that goes~ – Cat Plus Plus Jan 27 '13 at 03:04
@Erogol: "Ban me out" isn't English. 1 year, 10 months on Stack Overflow should be enough time to learn English. – Lightness Races in Orbit Jan 27 '13 at 03:12

score 4 · Accepted Answer · answered Jan 27 '13 at 00:30

4

This works only for static arrays, defined in the current scope.

All you get in your example is the size of a pointer to float divided by the size of float.

answered Jan 27 '13 at 00:30

nameless

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sizeof works unusual at c++ after array creation

2 Answers2