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Under C any pointer is already a pointer to an array it's built into the language along with pointer arithmetic ... you need to just learn that.
Literally declare any pointer of anything lets do a float
float* p;
now you can access it as an array
p[0] = 5.0;
p[100] = 10.0;
It will crash because the pointer isn't really to any memory but it makes the point the pointer is already a pointer to an array
There are no exceptions to the rule it doesn't matter if the pointer is to a fundamental type or struct .... so I don't get how you could ever forget that. In the C community the [] use is rare because it's two extra characters to type. It also has implication when declaring variables because it puts that array on the stack not on data memory or constant memory (rodata) if it determines its a constant. So if you get into the habit of using that form you can get some undesirable things happen.
Personally you are learning and I would learn to live without it and just learn them as you will most often see them written.
The topic is well covered in dummies guide to C but you will note the last statement
How to Use Arrays and Functions Together in C Programming - dummies[^]
In vino veritas
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I made a mistake. See Richard's post below.
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noticed
modified 29-Mar-20 12:16pm.
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Greg Utas wrote: char* somedata[]
That's an array of pointers.
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good pick didn't even notice the * because that form is so foreign to me.
In vino veritas
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Right you are! I forget to remove the *.
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Why not leave the * but remove the []? The parameter is a pointer to an array, not an actual array.
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I use brackets to indicate that the underlying is an array, not a pointer to a single char. Maybe it's because I was a latecomer to C++ and never used C idioms, another one being if(p), for which I write if(p != nullptr).
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A pointer rarely means a pointer to a single item, it always indicates a pointer to a set of items. If you want to pass a single int, char etc, then why use a pointer?
I would agree with you on using the if(p != nullptr) construct, it makes it much clearer.
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Richard MacCutchan wrote: If you want to pass a single int, char etc, then why use a pointer?
Because it might inadvertently be nullptr, and I find this defensive code jarring:
void f(type& t)
{
if(&t ! nullptr)...
}The optimize-everything crowd won't agree, but in my opinion code that invokes the above with a null reference should suffer a SIGSEGV before the function is called. But since that's not the case...
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I don't think a reference can ever be null.
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It's true that C++ has no explicit notion of a null reference. But if you run this
void test(int& i)
{
if(i == 1)
std::cout << i << '\n';
}
int main(int argc, char* argv[])
{
int* pi = nullptr;
test(*pi);
}it will SIGSEGV on the line if(i == 1). That's in a VS2017 debug build.
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That is interesting. It should really crash at the test(*pi); line, since it is trying to dereference a null pointer. I would also suggest the the compiler should recognise that pi is a pointer and not a reference.
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I agree that it should crash there. But I've never seen it work that way, though for most of my career I worked in a language where it would have crashed there.
It's not unusual to dereference a pointer (pi) and pass it to an argument that wants a reference.
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Greg Utas wrote: It's not unusual ...
Interesting, but not something I have ever done. I had (naively) assumed that the whole point of references was to avoid this very trap. Incidentally I tried it in g++ as well and the gave a SEGV.
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Where did it die in g++? Before or after calling the function?
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Same as in Windows, on the if statement in test function.
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Ditto for clang 9.0.1 (Fedora fc31)
Keep Calm and Carry On
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Richard MacCutchan wrote: f you want to pass a single int, char etc, then why use a pointer?
On occasion you want an "out" or sentinel parameter, so in those cases you have to use a pointer (or a reference if using C++).
There's lots of cases where you might have a pointer to a single struct that you either want to fill in, or avoid copying the whole thing to the stack. For the latter, of course, you'd mark it as const.
Keep Calm and Carry On
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how do you declare and access an array of pointers? if you want to cycle through pointers of same type in a for loop.
int * somedata[] = new int * [5]; ??
"DreamLand Page" on facebook
modified 29-Mar-20 14:42pm.
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In that case you'd use another level of indirection: e.g.
#include <iostream>
void myfn(int **data, size_t len)
{
for(size_t i = 0; i < len; ++i)
*data[i] = i * 2;
}
int main)_
{
int data[5] = { 1, 2, 3, 4, 5 }; const size_t ndata = sizeof(data)/sizeof(data[0]);
int** pdata = new int*[ndata];
for(size_t i = 0; i < ndata; ++i)
pdata[i] = &data[i];
std::cout << "Before:\n";
for(size_t i = 0; i < ndata; ++i)
std::cout <^lt; *pdata[i] << std::endl;
myfn(pdata, 5);
std::cout <*lt; "\nAfter:\n";
for(size_t i = 0; i < ndata; ++i)
std::cout << *pdata[i] << std::endl;
delete[] pdata;
return 0;
}
Keep Calm and Carry On
modified 29-Mar-20 17:49pm.
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This array of pointers thing is above trivial. Here is my old code:
CUSTOMVERTEX* ScreenLetters;
ScreenTextBuffers[0]->Lock( 0, 0, (void**)&ScreenLetters, 0 );
int LetterVertexDataincrement = 0;
int letterwidth = 12;
int letterheight = 12;
for(int ii = 0; ii < ScreenLetterGroups[0].height;ii++)
{
for(int i = 0; i < ScreenLetterGroups[0].width;i++)
{
ScreenLetters[LetterVertexDataincrement].position.x = i * letterwidth + ScreenLetterGroups[0].x;
ScreenLetters[LetterVertexDataincrement].position.y = ii * letterheight + ScreenLetterGroups[0].y;
ScreenLetters[LetterVertexDataincrement].position.z = 20;
ScreenLetters[LetterVertexDataincrement].color = 0xffffffff;
}
}
ScreenTextBuffers[0]->Unlock();
I have this piece of code repeating for every ScreenTextBuffers element. I want to place it in a `for` loop so what I`m doing is:
CUSTOMVERTEX ** ScreenLettersP_s = new CUSTOMVERTEX* [NumberOfTextBuffers];
int LetterVertexDataincrement = 0;
int letterwidth = 12;
int letterheight = 12;
for(int iii =0; iii < NumberOfTextBuffers; iii++)
{
ScreenTextBuffers[iii]->Lock( 0, 0, (void**)&ScreenLettersP_s[iii], 0 );
for(int ii = 0; ii < ScreenLetterGroups[iii].height;ii++)
{
for(int i = 0; i < ScreenLetterGroups[iii].width;i++)
{
*ScreenLettersP_s[LetterVertexDataincrement]->position.x = i * letterwidth + ScreenLetterGroups[iii].x;
*ScreenLettersP_s[LetterVertexDataincrement]->position.y = ii * letterheight + ScreenLetterGroups[iii].y;
*ScreenLettersP_s[LetterVertexDataincrement]->position.z = 20;
*ScreenLettersP_s[LetterVertexDataincrement]->color = 0xffffffff;
}
}
ScreenTextBuffers[i]->Unlock();
}
but it doesn`t compile, I`m getting an `illegal indirection` error
struct CUSTOMVERTEX
{
D3DXVECTOR3 position;
D3DCOLOR color;
FLOAT tu, tv;
};
modified 1-Apr-20 6:03am.
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Arrays of pointers are just as trivial as arrays of anything, if you understand how to address them.
fearless_ wrote: I`m getting an `illegal indirection` error
Since we cannot see your screen we also cannot guess where that occurs. Please format your code properly and explain exactly where the error occurs.
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