invalid conversion from void* to strcut* in c++

Question

when i try to use calloc for continuous memory allocation it gives me error like.. invalid conversion from void* to slotstruct(*)[100][1500] in c++

here is my code :

typedef struct
 {
   int id;
   bool used;
 }slotstruct;
 int main(){
       slotstruct ( *slot1 )[100][1500];
       slot1 = calloc( 1, 3 * sizeof( *slot1 ) );
      for(i=0;i<3;i++){
         for(j=0;j<100;j++){
            for(k=0;k<1500;k++){
                  cout << "Addresses are : "<<slot1[i][j][k];
           }
      }
    }
 }

Your code can not compile, is it really the code you get error? — apple apple, Sep 09 '16 at 05:30
@appleapple Read the first sentence in the question. The code not compiling is *exactly* why this was posted. — WhozCraig, Sep 09 '16 at 05:32
@WhozCraig no, it not compile for a lot other reason than what the OP ask — apple apple, Sep 09 '16 at 05:32
Off topic: `typedef struct {} slotstruct` is unnecessary in C++ . `struct slotstruct` would be sufficient. — user4581301, Sep 09 '16 at 05:34
@WhozCraig example: `i,j,k` is not declared, no `include`s and `using`s, finally what the OP ask for — apple apple, Sep 09 '16 at 05:34
if this is really the code OP use, the first error would be `calloc was not declared in this scope` or something like — apple apple, Sep 09 '16 at 05:38
you should use `new` and `delete` in c++ programs unless you have good reasons — apple apple, Sep 09 '16 at 05:43
Also off topic: `calloc` is dangerous in C++. It will not invoke constructors. It works here because your structure is trivial, but be warned. In C++ one cannot simply assign `void *` You must state you are deliberately doing something potentially stupid with a cast. — user4581301, Sep 09 '16 at 05:46
@appleapple OP isn't trying to output `slotstruct`. OP wants the address but neglected to use the address of operator. — user4581301, Sep 09 '16 at 05:57
Don't too much conversation here.. Use chat room for long conversation. — Balagurunathan Marimuthu, Sep 09 '16 at 07:10

WhozCraig · Accepted Answer · 2016-09-09T08:16:29.337

The C language allows implicit conversion (without casts) of void* to any other object pointer type. Such is not the case in C++. Your options are:

Don't compile C code with a C++ compiler (easiest).
Perform the proper cast to avoid the error (ill advised, casts, especially for beginners, are often used to hide problems rather than solve them. Abuse of them for that very purpose is, unfortunately, not uncommon. It would, however, "solve" your issue here).
Use new[] and delete[] for your sequence allocation. No casting is required, and if your data type ever becomes non-trivial, it will still work.
Don't use manual memory allocation at all, and instead opt for an RAII approach.

The first of these is obvious, the rest are shown below

Using a cast

Will work in this case with no ill effects because your slotstruct type is trivial:

slot1 = (slotstruct (*)[100][1500])calloc( 1, 3 * sizeof( *slot1 ) ); // allocates
free(slot1); // destroys

Using new[] (and delete[])

No cast required:

slot1 = new slotstruct[3][100][1500]; // allocates..
delete [] slot1; //.. destroys

C++ Alternative using RAII

A more proper C++ RAII approach for what you appear to be trying to accomplish looks like this:

#include <iostream>
#include <array>
#include <vector>

struct slotstruct
{
    int id;
    bool used;
};

int main()
{
    std::vector<std::array<std::array<slotstruct,1500>, 100>> slots(3);

    for (auto const& x : slots)
        for (auto const& y : x)
            for (auto const& z : y)
                std::cout <<"Address is : " << static_cast<const void*>(&z) << '\n';
}

Output (varies)

Address is : 0x100200000
Address is : 0x100200008
Address is : 0x100200010
Address is : 0x100200018
Address is : 0x100200020
...
Address is : 0x10056ee60
Address is : 0x10056ee68
Address is : 0x10056ee70
Address is : 0x10056ee78

Man. I wish I'd seen this go up. Would have gone to bed half an hour ago. Only thing I'd add is a quick note on why `'\n'` and not `endl`, and I'd change the inner most `auto const& x` to `auto const& z` or similar to prevent confusion over the duplicated variable name. — user4581301, Sep 09 '16 at 06:46
@user4581301 duh. I totally spaced on the `z`. The '\n' is, in a continual effort, to remind myself that unnecessary direct flushing requests is something that should be avoided. Sutter and Stroustrup talk about it briefly in the [Core C++ Guidelines](https://github.com/isocpp/CppCoreGuidelines/blob/master/CppCoreGuidelines.md#Rio-endl). Thanks the for catching the innermost index. — WhozCraig, Sep 09 '16 at 16:47

apple apple · Answer 2 · 2016-09-09T06:03:34.513

0

Only for what you ask: You should explicit do the type cast from void* to others in c++ programs.

And you should consider using new and delete instead.

edited Sep 09 '16 at 06:03

answered Sep 09 '16 at 05:50

apple apple

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invalid conversion from void* to strcut* in c++

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