When it's defined as a constant pointer, timers is an lvalue. An lvalue is an expression that designates an object. An rvalue is an expression that is not an lvalue. Since an rvalue does not necessarily refer to an object, compilers may be able to avoid generating data storage for rvalue expressions.
Avoiding generating storage for lvalues is harder, but not impossible. In C, constant objects declared at global scope have external linkage by default, as if they were declared with the keyword extern. This means that references to timers may appear in other translation units, and a C compiler must generate storage for timers just in case such external references exist. This means that all references to timers must appear in the same translation unit as the definition for timers.
In that case, the compiler might be able to determine that it doesn't need to generate the storage for the constant pointer. A C compiler should also be able to eliminate the storage for the constant pointer if you define it with the keyword static.
The first test program appears in Listing 1. It defines timers as a macro. In all cases, the compiler generated code that used immediate operands for the pointer values and didn't generate a copy of the constant pointer in the data space. Listing 1: A little test to see how the compiler generates code to access memory-mapped device registers.
Last time, I reported that this produced a change for one compiler. I've since reviewed my results and found I was mistaken. The C compilers should have been able to exploit this change to produce better code, but only one compiler actually took advantage of it. A missed observation Last time, I wrote that:. That is, they behave as if they had been declared with the keyword extern.
This means that references to timers may appear in other translation units and a C compiler must generate storage for timers just in case such external references exist. In theory, the linker might be able to determine that no external references exist and eliminate the storage for timers , but I don't know of a linker that does. Dave Baker wrote that he uses a C compiler that discards unused objects at link time.
I realized that, in doing my analysis last time, I had looked only at the generated assembly code, not at the linked executable programs. When I looked at the link maps for each test program, I found that one of the C compilers I had tested came with a linker that also discarded unused pointers.
Using a local pointer Thus far, all of the tested variations declare timers as a non-local name, either as a macro or a global. However, as a general rule, you should declare names in the smallest scope possible. There is a function to find the size of the map,ie,number of entries in the map, but is there any such method for the memory. I have a map string, string. The sizeof is always giving me a size of Any reason why this is so? Thanks :. There is no easy way, but if you really must know though If you are not happy with the default allocator for whatever reason, you can provide a custom allocator to the container template, and it will just seamlessly use that one.
Add to that the value of sizeof to be super precise. No, there is not. However you can achieve something similar for classes that support a. If you want to know the allocated memory you could use. The size of the map class is The instance for the map will be created in the stack and whatever records we insert will be stored in the heap.
So map object will be just pointing to the records in the heap. The doubt may be like why it is 48 even after inserting records.? As the records are not stored along with map object the size is constant - As mentioned here in the answers object size won't change in the run time. See the article Collections for an illustration of the derivation of a special-purpose list class.
This is a nested structure within class CMap. For an example of usage, see the example for CMap::PLookup. See the example for CMap::Lookup. Retrieves the map element at rNextPosition , then updates rNextPosition to refer to the next element in the map. This function is most useful for iterating through all the elements in the map. Note that the position sequence is not necessarily the same as the key value sequence.
The iteration sequence is not predictable; therefore, the "first element in the map" has no special significance. For best performance, the hash table size should be a prime number. To minimize collisions, the size should be roughly 20 percent larger than the largest anticipated data set.
See the example for CMap::RemoveAll. Lookup uses a hashing algorithm to quickly find the map element with a key that exactly matches the given key. Xavier Nodet Xavier Nodet 4, 2 2 gold badges 34 34 silver badges 48 48 bronze badges. Thats the only way to get an accurate number. Drakosha: No, that allocator is not meant to do this. Which is why you need to create a custom one that will report whatever you want it to.
Stephan T. Lavavej also has a nice article on custom allocators: blogs. Now I can feel justified in my continued use of the. Michael: Why headers all of sudden?
Some context will be of help! Personally, I found this to be surprisingly poor memory efficiency, but it is what it is. Hope this makes for a handy rule-of-thumb. In you need better memory efficiency, you could go with code. See my comment in Diomidis Spinellis answer.
The fraction of overhead will of course depend on the sizes of your keys and values. I am expanding on his answer by adding few lines of code.
Why do you need this size? I need this size to know hom much entries i can hold in memory before i start swapping. I need upper bound. If you need the upper bound, I suggest to take a look to the implementation of the map, to see how a map node object is implemented and to consider the size of the members of the map node as extra to the size of the key and element. Be aware that this approach is platform and compiler specific.
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