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POCopying big objects, c++
 

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  1. POCopying big objects, c++
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    <p>I am sure this has been answered, but I am no programmer and couldn't find/understand the appropriate answer. Suppose I have a massive class <code>big</code> for which would like to overload a binary operator, say <code>operator +</code>. </p> <ol> <li>Is there a sane way to make <code>big X=Y+Z</code> to build the sum directly into <code>X</code>, instead of creating temporary object, copying it to <code>X</code> and then destroying the temporary?</li> <li>The only thing that come into my mind is to wrap <code>big</code> into another class <code>small</code> that will contain the pointer to <code>big</code> and add <code>int use;</code> number of references to to big, so that <code>big</code> object get destroyed when <code>use==0</code>. And add another assignment operator, say <code>&lt;=</code> for actual copying. I tried to implement it (below). It seems to work, but I have no experience and it is hard for me to foresee, what can go wrong. Shouldn't be there a simpler solution?</li> </ol> <p>Code:</p> <pre><code>#include &lt;iostream&gt; // print and execute cmd #define Do(cmd) cout &lt;&lt; "\n\n\t"&lt;&lt; ++line &lt;&lt; ".\t" &lt;&lt; #cmd &lt;&lt; ";\n" &lt;&lt; endl; cmd; // print small object: name(small.id[big.id,u=big.use,x=big.x]) #define Show(avar) cout &lt;&lt; #avar &lt;&lt; "(" &lt;&lt; (avar).id &lt;&lt; "[" &lt;&lt; ((avar).data==NULL?0:(avar).data-&gt;id) &lt;&lt; ",u=" &lt;&lt; ((avar).data==NULL?0:(avar).data-&gt;use) &lt;&lt; ",x=" &lt;&lt; ((avar).data==NULL?0:(avar).data-&gt;x) &lt;&lt; "])" using namespace std; class big{ public: static int N; // biggest id in use int id; // unique id for each object int use; // nuber of references to this object int x; // data big() __attribute__((noinline)) { id=++N; use=1; x=0; cout &lt;&lt; "big.constructor.def: [" &lt;&lt; id &lt;&lt; ",u=" &lt;&lt; use &lt;&lt; ",x="&lt;&lt;x&lt;&lt;"]" &lt;&lt; endl; } big(const int&amp; y) __attribute__((noinline)) { id=++N; x=y; use=1; cout &lt;&lt; "big.constructor.int: [" &lt;&lt; id &lt;&lt; ",u=" &lt;&lt; use &lt;&lt; ",x="&lt;&lt;x&lt;&lt;"]" &lt;&lt; endl; } big(const big&amp; b) __attribute__((noinline)) { id=++N; use=1; x=b.x; cout &lt;&lt; "big.constructor.copy: [" &lt;&lt; id &lt;&lt; ",u=" &lt;&lt; use &lt;&lt; ",x="&lt;&lt;x&lt;&lt;"]" &lt;&lt; endl; } ~big() __attribute__((noinline)) { if(use&gt;0) throw 99; // destroing referenced data! cout &lt;&lt; "big.destructor: [" &lt;&lt; id &lt;&lt; ",u=" &lt;&lt; use &lt;&lt; ",x="&lt;&lt;x&lt;&lt;"]" &lt;&lt; endl; } friend class small; }; class small{ public: static int N; // biggest id in use int id; // unique id big * data; // reference to the actual data small() __attribute__((noinline)) { id=++N; data=NULL; // contains no data cout &lt;&lt; "small.constructor.def: "; Show(*this)&lt;&lt; endl; } small(const int&amp; y) __attribute__((noinline)) { id=++N; data=new big (y); // relies on the big constructor cout &lt;&lt; "small.constructor.int: "; Show(*this)&lt;&lt;endl; } small(const small&amp; y) __attribute__((noinline)) { id=++N; data=y.data; // new object refers to the same data!! if(data!=NULL) ++(data-&gt;use); // new reference added; cout &lt;&lt; "small.constructor.copy: "; Show(y) &lt;&lt; "--&gt;"; Show(*this) &lt;&lt; endl; } ~small(){ cout &lt;&lt; "small.destructor: "; Show(*this)&lt;&lt; endl; if(data!=NULL){ // is there data? --(data-&gt;use); // one reference is destroyed if(data-&gt;use == 0) // no references left, kill the data delete data; } } const small&amp; operator= (const small&amp; b) __attribute__((noinline)) { cout &lt;&lt; "equal: "; Show(*this) &lt;&lt; " = "; Show(b)&lt;&lt;endl; if(data != NULL){ // is there data in the target? --(data-&gt;use); // one reference is destroyed if(data-&gt;use == 0) // no references left, delete data; // kill the data } data=b.data; // target referenses the same data as the source! if(data!=NULL) ++(data-&gt;use); // new references added cout &lt;&lt; "Done equal: "&lt;&lt;endl; return *this; } // &lt;= will be used for actual copying the data const small&amp; operator&lt;= (const small&amp; b) __attribute__((noinline)) { cout &lt;&lt; "Copy: "; Show(*this) &lt;&lt; " &lt;= "; Show(b)&lt;&lt;endl; if(data != NULL){ // is there data in the target? --(data-&gt;use); // one reference is destroyed if(data-&gt;use == 0) // no references left, delete data; // kill the data } if(b.data==NULL) // source has no data data=NULL; else data = new big(*(b.data)); // make new copy of the data // via big's copy constructor cout &lt;&lt; "Done copy: "&lt;&lt;endl; return *this; } small operator+ (const small&amp; b) __attribute__((noinline)) { cout &lt;&lt; "Plus: "; Show(*this) &lt;&lt; " + "; Show(b)&lt;&lt; endl; if(this-&gt;data == NULL | b.data == NULL) throw 99; // missing data for + small ret(data-&gt;x); ret.data-&gt;x += b.data-&gt;x; cout &lt;&lt; "Return: "; Show(ret)&lt;&lt;endl; return ret; } }; int big::N=0; int small::N=0; main(){ int line=0; Do(small X(5); small Y(6); small Z(7); small W(X)); Show(X) &lt;&lt; endl; Show(Y) &lt;&lt; endl; Show(Z) &lt;&lt; endl; Show(W) &lt;&lt; endl; Do(X=Y; Z&lt;=Y); Show(X)&lt;&lt;endl; Show(Y)&lt;&lt;endl; // X and Y refer to the same data Show(Z)&lt;&lt;endl; // Z has a copy of data in Y Do(X=Z; Y=Z); Show(X)&lt;&lt;endl; Show(Y)&lt;&lt;endl; Show(Z)&lt;&lt;endl; // data previosly in X,Y destroyed Do(small* U=new small (17); small* T=new small (*U)); Show(*U) &lt;&lt; endl; Show(*T) &lt;&lt; endl; // U and T refer to the same big Do(delete U); Show(*T) &lt;&lt; endl; // big stays since there is another reference to it Do(delete T); // big destroyed Do(X=(Y+Z)+W); Show(X)&lt;&lt;endl; Show(Y)&lt;&lt;endl; Show(Z)&lt;&lt;endl; // no extra copying of data occures cout &lt;&lt; "\n\tEND\n" &lt;&lt; endl; } </code></pre> <p>Output:</p> <pre><code>1. small X(5); small Y(6); small Z(7); small W(X); big.constructor.int: [1,u=1,x=5] small.constructor.int: *this(1[1,u=1,x=5]) big.constructor.int: [2,u=1,x=6] small.constructor.int: *this(2[2,u=1,x=6]) big.constructor.int: [3,u=1,x=7] small.constructor.int: *this(3[3,u=1,x=7]) small.constructor.copy: y(1[1,u=2,x=5])--&gt;*this(4[1,u=2,x=5]) X(1[1,u=2,x=5]) Y(2[2,u=1,x=6]) Z(3[3,u=1,x=7]) W(4[1,u=2,x=5]) 2. X=Y; Z&lt;=Y; equal: *this(1[1,u=2,x=5]) = b(2[2,u=1,x=6]) Done equal: Copy: *this(3[3,u=1,x=7]) &lt;= b(2[2,u=2,x=6]) big.destructor: [3,u=0,x=7] big.constructor.copy: [4,u=1,x=6] Done copy: X(1[2,u=2,x=6]) Y(2[2,u=2,x=6]) Z(3[4,u=1,x=6]) 3. X=Z; Y=Z; equal: *this(1[2,u=2,x=6]) = b(3[4,u=1,x=6]) Done equal: equal: *this(2[2,u=1,x=6]) = b(3[4,u=2,x=6]) big.destructor: [2,u=0,x=6] Done equal: X(1[4,u=3,x=6]) Y(2[4,u=3,x=6]) Z(3[4,u=3,x=6]) 4. small* U=new small (17); small* T=new small (*U); big.constructor.int: [5,u=1,x=17] small.constructor.int: *this(5[5,u=1,x=17]) small.constructor.copy: y(5[5,u=2,x=17])--&gt;*this(6[5,u=2,x=17]) *U(5[5,u=2,x=17]) *T(6[5,u=2,x=17]) 5. delete U; small.destructor: *this(5[5,u=2,x=17]) *T(6[5,u=1,x=17]) 6. delete T; small.destructor: *this(6[5,u=1,x=17]) big.destructor: [5,u=0,x=17] 7. X=(Y+Z)+W; Plus: *this(2[4,u=3,x=6]) + b(3[4,u=3,x=6]) big.constructor.int: [6,u=1,x=6] small.constructor.int: *this(7[6,u=1,x=6]) Return: ret(7[6,u=1,x=12]) Plus: *this(7[6,u=1,x=12]) + b(4[1,u=1,x=5]) big.constructor.int: [7,u=1,x=12] small.constructor.int: *this(8[7,u=1,x=12]) Return: ret(8[7,u=1,x=17]) equal: *this(1[4,u=3,x=6]) = b(8[7,u=1,x=17]) Done equal: small.destructor: *this(8[7,u=2,x=17]) small.destructor: *this(7[6,u=1,x=12]) big.destructor: [6,u=0,x=12] X(1[7,u=1,x=17]) Y(2[4,u=2,x=6]) Z(3[4,u=2,x=6]) END small.destructor: *this(4[1,u=1,x=5]) big.destructor: [1,u=0,x=5] small.destructor: *this(3[4,u=2,x=6]) small.destructor: *this(2[4,u=1,x=6]) big.destructor: [4,u=0,x=6] small.destructor: *this(1[7,u=1,x=17]) big.destructor: [7,u=0,x=17] </code></pre>
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    1. COAre you positive the optimizer doesn't build the answer directly in `X`? Return value optimization?
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    2. COThank you all who bothered, especially @juanchopanza, this Don Quixote wishes to have such one. g++ -O2 does the job,if function returns the same local with all return statements. "Move semantics" would probably do the job too, as far as I can see.
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