I need to create a dynamically-allocated array of const objects. What makes it difficult is that I need to have values assigned to the const objects too.
I need this for Samples variable of this SFML class.
How should I do it?
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As @Mike Seymour figured out, you probably don't understand what you need correctly.Kerrek SB– Kerrek SB2011-11-03 17:00:01 +00:00Commented Nov 3, 2011 at 17:00
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5 Answers
You don't need an array of const objects. A pointer-to-const can point to either const or non-const objects; you can create a dynamic array and initialise a Chunk structure from it like this:
std::vector<Int16> samples;
initialise(samples);
// valid until 'samples' is destroyed or resized
SoundStream::Chunk chunk = {&samples[0], samples.size()};
2 Comments
Kerrek SB
Ahh, someone who read up the background to the OP's question ... and unsurprisingly discovered that the OP didn't know what she needed! +1 for that. (The Fundamental Problem of Life is seeing through what people say they want and discover what they need. I think the Rolling Stones figured that out way before usability analysts became popular.)
sim642
Thanks for this nice STL solution.
Easy:
// Step 1: Make an array of const values:
const int arr[] = { 1, 4, 9, 17 };
// Step 2: Make a pointer to it:
auto parr = &arr; // 2011-style
const int (*pbrr)[4] = &arr; // old-style
You cannot "assign" values to constants (obviously), so the only way to endow a constant with a value is to initialize it to that value.
4 Comments
Julien-L
What is the purpose of the parenthesis around
*pbrr? Thanks!sim642
Sorry, forgot to add that I need the array to be dynamically-allocated.
Emile Cormier
Step 3: Profit! (sorry, old cliché)
Kerrek SB
@sim642: A dynamic array of constants is only possible in C++11, since you need uniform initialization to initialize it:
const int * p = new int[3] { 1, 2, 3 };.Or, if the data is not known at compile time:
const std::vector<int> function() {
std::vector<int> tmp(5); //make the array
for(int i=0; i<5; ++i)
tmp [i] = i; //fill the array
return tmp;
}
Comments
Do the allocation, assign it to a pointer to non-const. Make your modifications to the data. When you're done muckin' things about, then you can assign your const pointer to the array. For example:
int * p = new int[100];
for (int i=0; i<100; ++i)
p[i] = i;
const int * cp = p;
Comments
Should you need a dynamically allocated array, I recommend using a standard container:
std::vector<Int16> data;
Chunk* c = ...;
data.push_back(...);
c->Samples = &data[0];
1 Comment
sim642
Sorry, forgot to add that I need the array to be dynamically-allocated.