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Memory in C++ Sizing up Memory the gamedesigninitiative at cornell university Lecture 13 Memory in C++ Sizing Up Memory Primitive Data Types Complex Data Types byte: basic value (8 bits) Pointer: platform dependent 4 bytes on 32 bit machine char: 1 byte 8 bytes on 64 bit machine short: 2 bytes Java reference is a pointer int: 4 bytes Array: data size * length Not standard long: 8 bytes May change Strings same (w/ trailing null) Struct: sum of fields float: 4 bytes IEEE standard Same rule for classes Won’t change double: 8 bytes Structs = classes w/o methods the gamedesigninitiative 2 Memory in C++ at cornell university Memory Example class Date { short year; 2 byte byte day; 1 byte byte month; 1 bytes } 4 bytes class Student { int id; 4 bytes Date birthdate; 4 bytes Student* roommate; 4 or 8 bytes (32 or 64 bit) } 12 or 16 bytes the gamedesigninitiative 3 Memory in C++ at cornell university Memory and Pointer Casting C++ allows ANY cast // Floats for OpenGL Is not “strongly typed” float[] lineseg = {0.0f, 0.0f, 2.0f, 1.0f}; Assumes you know best But must be explicit cast // Points for calculation Vec2* points Safe = aligns properly // Convert to the other type Type should be same size points = (Vec2*)lineseg; Or if array, multiple of size for(int ii = 0; ii < 2; ii++) { Unsafe = data corruption CULog("Point %4.2, %4.2", It is all your fault points[ii].x, points[ii].y); Large cause of seg faults } the gamedesigninitiative 4 Memory in C++ at cornell university Two Main Concerns with Memory Allocating Memory With OS support: standard allocation Reserved memory: memory pools Getting rid of memory you no longer want Doing it yourself: deallocation Runtime support: garbage collection the gamedesigninitiative 5 Memory in C++ at cornell university C/C++: Allocation Process malloc new Based on memory size Based on data type Give it number of bytes Give it a data type Typecast result to assign it If a class, calls constructor No initialization at all Else no default initialization Example: Example: char* p = (char*)malloc(4) Point* p = new Point(); Stack Heap Stack Heap sizeof ? n bytes 1 ? 0 (Class) … … ? 1 the gamedesigninitiative 6 Memory in C++ at cornell university C/C++: Allocation Process malloc new Based on memory size Based on data type Give it number of bytes Give it a data type Typecast result to assign it If a class, calls constructor No initialization at all Else no default initialization Preferred in C Preferred in C++ Example: Example: char* p = (char*)malloc(4) Point* p = new Point(); Stack Heap Stack Heap sizeof ? n bytes 1 ? 0 (Class) … … ? 1 the gamedesigninitiative 7 Memory in C++ at cornell university Custom Allocators Pre-allocated Array (called Object Pool) Start Free End Idea: Instead of new, get object from array Just reassign all of the fields Use Factory pattern for constructor Easy if only See alloc() method in CUGL objects one object Problem: Running out of objects type to We want to reuse the older objects allocate Easy if deletion is FIFO, but often isn’t the gamedesigninitiative 8 Memory in C++ at cornell university Custom Allocators in CUGL class Texture : : public enable_shared_from_this<Texture> { public: /** Creates a sprite with an image filename. */ static shared_ptr<Texture> allocWithFile(const string& file); Allocation & initialization /** Creates a sprite with a Texture2D object. */ static shared_ptr< Texture> allocWithData(const void *data, int w, int h); private: /** Creates, but does not initialize sprite */ Allocation Texture(); only /** Initializes a sprite with an image filename. */ virtual bool initWithFile(const string& file); Initialization /** Initializes a sprite with a texture. */ only virtual bool initWithData(const void *data, int w, int h); }; the gamedesigninitiative 9 Memory in C++ at cornell university Custom Allocators in CUGL class Texture : : public enable_shared_from_this<Texture> { public: /** Creates a sprite with an image filename. */ static shared_ptr<Texture> allocWithFile(const string& file); Allocation & initialization /** CreatesCustomizable a sprite with a Texture2D allocation object. */ static shared_ptr< Texture> allocWithData(const void *data, int w, int h); private: /** Creates, but does not initialize sprite */ Allocation Sprite(); Standard allocation only /** Initializes a sprite with an image filename. */ virtual bool initWithFile(const string& file); Initialization /** Initializes a sprite with a texture. */ only virtual bool initWithData(const void *data, int w, int h); }; the gamedesigninitiative 10 Memory in C++ at cornell university Free Lists Create an object queue // Free the new particle Separate from preallocation freelist.push_back(p); Stores objects when “freed” … To allocate an object… // Allocate a new particle Look at front of free list Particle* q; If object there take it if (!freelist.isEmpty()) { Otherwise make new object q = freelist.pop(); Preallocation unnecessary } else { Queue wins in long term q = new Particle(); Main performance hit is } deletion/fragmentation q.set(…) the gamedesigninitiative 11 Memory in C++ at cornell university Particle Pool Example the gamedesigninitiative 12 Memory in C++ at cornell university Particle Pool Example See FreeList and GreedyFreeList the gamedesigninitiative 13 Memory in C++ at cornell university Two Main Concerns with Memory Allocating Memory With OS support: standard allocation Reserved memory: memory pools Getting rid of memory you no longer want Doing it yourself: deallocation Runtime support: garbage collection the gamedesigninitiative 14 Memory in C++ at cornell university Manual Deletion in C/C++ Depends on allocation int main() { malloc: free cout << "Program started" << endl; int* a = new int[LENGTH]; new: delete What does deletion do? delete a; Marks memory as available for(int ii = 0; ii < LENGTH; ii++) { Does not erase contents cout << "a[" << ii << "]=" Does not reset pointer << a[ii] << endl; Only crashes if pointer bad } Pointer is currently NULL cout << "Program done" << endl; Pointer is illegal address } the gamedesigninitiative 15 Memory in C++ at cornell university Recall: Allocation and Deallocation Not An Array Arrays Basic format: Basic format: type* var = new type(params); type* var = new type[size]; … … delete var; delete[] var; // Different Example: Example: int* x = new int(4); int* array = new int[5]; Point* p = new Point(1,2,3); Point* p = new Point[7]; One you use the most Forget [] == memory leak the gamedesigninitiative 16 C++ Overview at cornell university Memory Leaks Leak: Cannot release memory Object allocated on heap Only reference is moved Consumes memory fast! Can even happen in Java memoryArea = newArea; JNI supports native libraries Method may allocate memory Need another method to free Example: dispose() in JOGL the gamedesigninitiative 17 Memory in C++ at cornell university A Question of Ownership void foo() { void foo(int key) { MyObject* o = MyObject* o = new MyObject(); table.get(key); o.doSomething(); o.doSomething(); o = null; o = null; Memory Not a return; Leak return; Leak } } the gamedesigninitiative 18 Memory in C++ at cornell university A Question of Ownership void foo() { void foo(int key) { MyObject* o = MyObject* o = table.get(key); table.get(key); table.remove(key); table.remove(key); ntable.put(key,o); o = null; Memory o = null; Leak? return; Not a return; Leak } } the gamedesigninitiative 19 Memory in C++ at cornell university A Question of Ownership Thread 1 Thread 2 “Owners” of obj void run() { void run() { o.doSomething1(); o.doSomething2(); } } Who deletes obj? the gamedesigninitiative 20 Memory in C++ at cornell university Understanding Ownership Function-Based Object-Based Object owned by a function Owned by another object Function allocated object Referenced by a field Can delete when function done Stored in a data structure Ownership never transferred Allows multiple ownership No guaranteed relationship May pass to other functions between owning objects But always returns to owner Call each owner a reference Really a stack-based object When can we deallocate? Active as long as allocator is No more references But allocated on heap (why?) References “unimportant” the gamedesigninitiative 21 Memory in C++ at cornell university Understanding Ownership Function-Based Object-Based Object owned by a function Owned by another object Function allocated object Referenced by a field Can delete when function done Stored in a data structure Ownership never transferred Allows multiple ownership No guaranteed relationship MayEasy pass: to Will other ignorefunctions between owning objects But always returns to owner Call each owner a reference Really a stack-based object When can we deallocate? Active as long as allocator is No more references But allocated on heap (why?) References “unimportant” the gamedesigninitiative 22 Memory in C++ at cornell university Reference Strength Strong Reference Weak Reference Reference asserts ownership Reference != ownership Cannot delete referred object Object can be deleted anytime Assign to NULL to release Often for performance caching Else assign to another object Only use indirect references Can use reference directly Copy to local variable first Compute on local variable No need to copy reference Treat like a normal object Be prepared for NULL Reconstruct the object? Standard type of reference Abort the computation? the gamedesigninitiative 23 Memory in C++ at cornell university
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