Safe to declare an array of structures that contain arrays?

This is a discussion on Safe to declare an array of structures that contain arrays? within the C Programming forums, part of the General Programming Boards category; Is it safe to declare an array of structures, if the strucure itself contains dynamically allocated arrays? Below is some ...

  1. #1
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    Safe to declare an array of structures that contain arrays?

    Is it safe to declare an array of structures, if the strucure itself contains dynamically allocated arrays? Below is some sample code.

    The program I'm working on seems to work, but I'm worried that the array of structures will get me in trouble later.

    Code:
    #include <stdio.h>
    
    
    struct MyStruct{
    
    	double V1;     
    	double V2;      
    	double * A1;  
    	double * A2; 		             
    
    };
    
    
    int main() {
    
    
    	int n = 10; 
    
    	struct MyStruct *x;
    	x = (struct MyStruct*)malloc((n)*sizeof(struct MyStruct));
    
    
    	for( i = 0; i < n; i++ ){			
    		x[i].A1 = (double*)malloc(60*sizeof(double));
    		x[i].A2 = (double*)malloc(60*sizeof(double));
    	}
    
    
    	//
    	//
    	// insert useful code here
    	//
    	//
    	    
    	    
    	for( i = 0; i < n; i++ ){
    		free(x[i].A1);
    		free(x[i].A2);
    	}	  	
    	
    	free(x);     	      
    
    
    return 0;       
    }

  2. #2
    a_capitalist_story
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    As long as you're fastidious in your memory cleanup you should be fine. However, you should not need to cast the return value of malloc if you're compiling this as C code, and doing so can hide problems.

  3. #3
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    Quote Originally Posted by rags_to_riches View Post
    As long as you're fastidious in your memory cleanup you should be fine. However, you should not need to cast the return value of malloc if you're compiling this as C code, and doing so can hide problems.
    Like including the proper header file.

  4. #4
    Frequently Quite Prolix dwks's Avatar
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    Also, a useful idiom is to use sizeof(*p) to determine the size of each block of memory you're allocating, instead of using a hard-coded type like sizeof(struct something). That way, if the type of p changes, the malloc() statements don't become out of date (which can be dangerous). For example, this
    Code:
    	struct MyStruct *x;
    	x = (struct MyStruct*)malloc((n)*sizeof(struct MyStruct));
    can be written as
    Code:
    	struct MyStruct *x = malloc(n * sizeof(*x));
    Never fear for efficiency: the sizeof() is still evaluated at compile-time, so it's just as efficient.
    dwk

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  5. #5
    ATH0 quzah's Avatar
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    Technically, you don't have a structure which has an array. You have a structure which has a pointer. There's a big difference. If it were an actual array, you wouldn't have to free the array (in fact, you could not free it implicitly, rather, you would free the actual structure--if you had allocated it dynamically).


    Quzah.
    Hope is the first step on the road to disappointment.

  6. #6
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    Quote Originally Posted by rags_to_riches View Post
    As long as you're fastidious in your memory cleanup you should be fine. However, you should not need to cast the return value of malloc if you're compiling this as C code, and doing so can hide problems.
    I think I'm actually compiling this as C++. I'm using mkoctfile to compile, and the file I'm using ends in .cc.

    When I tried to compile without the casting, I got some message about conversion from MyStruct* to void*.

  7. #7
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    Quote Originally Posted by quzah View Post
    Technically, you don't have a structure which has an array. You have a structure which has a pointer. There's a big difference. If it were an actual array, you wouldn't have to free the array (in fact, you could not free it implicitly, rather, you would free the actual structure--if you had allocated it dynamically).

    Quzah.
    That makes sense. It would explain why my program hasn't crashed.

  8. #8
    Frequently Quite Prolix dwks's Avatar
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    Yes, that's what happens in C++. If you're really writing C code, it's best to use .c files and compile as C. There are some subtle differences that can trip you up.

    This particular difference is because C allows pointers of any type to be assigned to and from void* pointers; but C++ only allows assignments to void* pointers. Assignments from void* pointers, like
    Code:
    char *p = malloc(10);
    need to have a cast in C++.

    There's an entry in the FAQ about this.
    dwk

    Seek and ye shall find. quaere et invenies.

    "Simplicity does not precede complexity, but follows it." -- Alan Perlis
    "Testing can only prove the presence of bugs, not their absence." -- Edsger Dijkstra
    "The only real mistake is the one from which we learn nothing." -- John Powell


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