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Closed 11 years ago.
Similar to this question, what are the pro/cons to using const local variables like in this answer?
Personally, I like to use const for any declared object (I'm not sure the word "variable" applies) unless I'm actually going to modify it. It documents to the reader that the value of the object is always going to be whatever it was initialized to, which can make the code easier to follow and analyze.
(It can also help the compiler in some cases, but most compilers, when invoked in optimizing mode, are smart enough to notice that the object is never modified. And if you invoke the compiler in non-optimizing mode, you're telling it that you don't care much about performance.)
In fact, if I were going to design my own language, all declared objects would be const (read-only) by default unless you explicitly mark them as modifiable.
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Closed 9 years ago.
I'm initializing an instance of PhysWorld class as shown here:
At this point the member variables are as follows:
This seems correct to me.
Then this line executes:
We step into:
And at this point, the member variables look like:
Can someone please help me understand what is going on here? This is one of my first attempts in c++ so I'm guessing it's something stupid on my part.
Thanks!
You probably loose variable value on assignment:
pw = PhysWorld(...);
This statement constructs a temporary object, and then makes a call: pw.operator=(const PhysWorld&);. Check how you implement it (if you do).
Also your function setRectDef contains a serious bug: you are storing a pointer to a stack variable, which would be invalid after leaving the function scope, and accessing it later most likely ruin your stack.
Edit: how to handle tmpS.
You need to allocate your structure on heap:
b2PolygoinShapre *tmpS = new b2PolygoinShape;
tmpS->SetAsTextBox(...);
this->rect = tmpS;
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Closed 9 years ago.
I have huge problems with making this code work properly.
http://pastebin.com/Mi6gj188
There's an output from example program on the bottom. It simply crashes and doesn't deliver proper results too. It seems that none of the overloaded operators work as it should
You didn't write a copy constructor, or use RAII. As a result, every time your vector object is copied (and it is, a lot, because you make no use of references!!) your internal data pointer is copied, sharing it amongst multiple objects (each of which will attempt to delete it on destruction) causing a horrible bug.
Your book tells you about the rule of three, which you should now go ahead and work on following.
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Closed 10 years ago.
In C and C++, what is the advantage of making a local const variable static? Assuming the initialization does not use other variables, is there any difference between preserving the value between calls, and setting the same constant value each call?
Could a valid C compiler ignore the static?
In C++, it avoids the construction/destruction between calls, but could there be any other benefit?
It doesn't take up stack-space may be a benefit if you have something like:
static const double table[fairly_large_number] = { .... };
Obviously, cost of construction can also be substantial enough that if the function is called a lot, there's good value in only constructing the object once.
Yes, and it is huge: a semantic benefit.
When you put const, you don't just mean the compiler shouldn't let you modify the variable. You make a bolder statement to whoever reads the code later: this won't ever change. Not even by a side effect where you give this variable as a pointer to another function.
Also, the compiler can take advantage of that new information and optimize it away in some situations, depending on the specific type you are dealing with.
(to be clear, I'm speaking here about const vs. non-const, not static vs. non-static.)
Edit: This SO answer is very informative too.
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Closed 10 years ago.
I have a strategy pattern, and would like to run it in a main loop in a game for example. The problem is there will be a memory leak If I'm not deleting the instance, and I also would like to use that instance somewhere else. How can I deal with memory allocation/deallocation in a strategy pattern.
CompressionContext *ctx = new CompressionContext();
//we could assume context is already set by preferences
ctx->setCompressionStrategy(new ZipCompressionStrategy());
//get a list of files
ctx->createArchive(fileList);
Use an std::shared_ptr<CompressionContextBase> instead of a CompressionContextBase* (i.e. a raw pointer).
Edit: This is just a suggestion, based on the information you provided, there may be other smart pointer implementations with different semantics, such as e.g. unique_ptr, which might be more suited. As #akappa suggests, you may want to read up on the topic more, to make a better decision -- again, based on the information in the question, you probably want a shared_ptr but there might be additional considerations you omitted.
It's difficult to tell what is being asked here. This question is ambiguous, vague, incomplete, overly broad, or rhetorical and cannot be reasonably answered in its current form. For help clarifying this question so that it can be reopened, visit the help center.
Closed 10 years ago.
I have come across loop-unrolling but what other types of compiler optimization are there for C++ code?
If possible, I'd be interested specifically for the Intel Compiler and GNU Compiler.
If I could obtain a list I can google for the explanation upon each type of optimization.
if you are talking generically, beyond loop unrolling, there is also the basic:
remove unchanging variables out of a loop.
optimizing away unused but initialized objects/variables/instances.(dead code removal)
expanding function calls in line, like strlen();
using processor specific directives/commands.
thats off the top of my head... I will be back with some scientific (wikipedia lol) answers
heres more:
5. static variable inlining
6. complex branch optimization
ok, tired lol heres a decent link i was just looking at :)
http://www.eetimes.com/electronics-products/embedded-tools/4086427/Advanced-Compiler-Optimization-Techniques