Passing an array as argument to a function
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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
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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
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; // assign value to address pointed to by data[i]// alternatively : // for(size\_t i = 0; i < len; ++i) // \*\*data++ = i\*2; // Note: use double de-reference and post increment!}
int main)_
{
int data[5] = { 1, 2, 3, 4, 5 }; // our original data
const size_t ndata = sizeof(data)/sizeof(data[0]);
int** pdata = new int*[ndata]; // double indirection used for definition of pdata// assign each element of pdata the address of element of data for(size\_t i = 0; i < ndata; ++i) pdata\[i\] = &data\[i\]; // or could use 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
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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
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I didn't know about
_countof(). Is it an MS only extension? Trying to compile with gcc under linux produces an implicit declaration warning in C and a not declared in this scope in C++Keep Calm and Carry On
As it is a macro it's easy to test for and if not there simply use a copy of the macro
#if !defined(_countof)
#define _countof(_Array) (sizeof(_Array) / sizeof(_Array[0]))
#endifIn vino veritas
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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
SIGSEGVbefore the function is called. But since that's not the case...Robust Services Core | Software Techniques for Lemmings | Articles
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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
SIGSEGVon the lineif(i == 1). That's in a VS2017 debug build.Robust Services Core | Software Techniques for Lemmings | Articles
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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
SIGSEGVon the lineif(i == 1). That's in a VS2017 debug build.Robust Services Core | Software Techniques for Lemmings | Articles
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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 thatpiis a pointer and not a reference.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.Robust Services Core | Software Techniques for Lemmings | Articles
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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.Robust Services Core | Software Techniques for Lemmings | Articles
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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.
Where did it die in g++? Before or after calling the function?
Robust Services Core | Software Techniques for Lemmings | Articles
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Where did it die in g++? Before or after calling the function?
Robust Services Core | Software Techniques for Lemmings | Articles
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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; // assign value to address pointed to by data[i]// alternatively : // for(size\_t i = 0; i < len; ++i) // \*\*data++ = i\*2; // Note: use double de-reference and post increment!}
int main)_
{
int data[5] = { 1, 2, 3, 4, 5 }; // our original data
const size_t ndata = sizeof(data)/sizeof(data[0]);
int** pdata = new int*[ndata]; // double indirection used for definition of pdata// assign each element of pdata the address of element of data for(size\_t i = 0; i < ndata; ++i) pdata\[i\] = &data\[i\]; // or could use 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
thanks k5054
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The documentation on MSDN (_countof Macro | Microsoft Docs[^]) does not identify it as MS only.
The leading '_' indicates it's a system level macro, i. e. Windows specific.
GOTOs are a bit like wire coat hangers: they tend to breed in the darkness, such that where there once were few, eventually there are many, and the program's architecture collapses beneath them. (Fran Poretto)
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Hi What is the syntax when you want to pass an array as argument? I`m looking for syntax for both function call and function definition.
You can do either of these, depending on what you want to do:
// pass read only array of ints
void process_Carray(int const* values, int n_values);
template // caution: this will create a separate function for each array size!
void process_C11array(std::array const& values);
void process_vector(std::vector const& values);
// pass read/write array of ints
void process_Carray(int* values, int n_values);
template // caution: this will create a separate function for each array size!
void process_C11array(std::array& values);
void process_vector(std::vector& values);The first variant is deprecated in C++, it should be restricted to pure C code. The second variant is useful if you know the size of your arrays at compile time (and it's always the same) The third variant is the most flexible as you don't need to know the array size, and you can even add more values within your function if you desire.
GOTOs are a bit like wire coat hangers: they tend to breed in the darkness, such that where there once were few, eventually there are many, and the program's architecture collapses beneath them. (Fran Poretto)
-
The leading '_' indicates it's a system level macro, i. e. Windows specific.
GOTOs are a bit like wire coat hangers: they tend to breed in the darkness, such that where there once were few, eventually there are many, and the program's architecture collapses beneath them. (Fran Poretto)
-
You can do either of these, depending on what you want to do:
// pass read only array of ints
void process_Carray(int const* values, int n_values);
template // caution: this will create a separate function for each array size!
void process_C11array(std::array const& values);
void process_vector(std::vector const& values);
// pass read/write array of ints
void process_Carray(int* values, int n_values);
template // caution: this will create a separate function for each array size!
void process_C11array(std::array& values);
void process_vector(std::vector& values);The first variant is deprecated in C++, it should be restricted to pure C code. The second variant is useful if you know the size of your arrays at compile time (and it's always the same) The third variant is the most flexible as you don't need to know the array size, and you can even add more values within your function if you desire.
GOTOs are a bit like wire coat hangers: they tend to breed in the darkness, such that where there once were few, eventually there are many, and the program's architecture collapses beneath them. (Fran Poretto)
thanks, I also understand `defensive programming` now.
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Much the same syntax as when passing anything, in this case a pointer is the norm:
int myFunction(char* anArrayOfChars) // a pointer to the array
{
// function code ...
}...
// calling code
char anArray[32];
// fill the array
int value = myFunction(anArray) // name of the array is a pointer to its contentSo is this pointer a save of the physical address of the real thing or just an artifice done by the compiler which matches the data behind the scenes to achieve the desired result?
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So is this pointer a save of the physical address of the real thing or just an artifice done by the compiler which matches the data behind the scenes to achieve the desired result?
It is the actual address in the pointer. In that way you can address any array, or any structure just by passing the real address to the function.
void myFunc(char* someData, int length)
{
for (int i = 0; i < length; ++i)
{
char c = toUpper(someData[i]);
someData[i] = c; // comnvert the string to upper case
}
}You can now call that function with any array of any length and get it converted. In every case the function receives the physical address of the array and accesses each character by using the index value
i, where0 <= i < length. -
It is the actual address in the pointer. In that way you can address any array, or any structure just by passing the real address to the function.
void myFunc(char* someData, int length)
{
for (int i = 0; i < length; ++i)
{
char c = toUpper(someData[i]);
someData[i] = c; // comnvert the string to upper case
}
}You can now call that function with any array of any length and get it converted. In every case the function receives the physical address of the array and accesses each character by using the index value
i, where0 <= i < length.thank you Richard
