Passing structures to a function

This is a discussion on Passing structures to a function within the C++ Programming forums, part of the General Programming Boards category; I have a class that contains a vector of structs. I want to pass each of the structs in the ...

  1. #1
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    Passing structures to a function

    I have a class that contains a vector of structs. I want to pass each of the structs in the array to a function one by one. I get a run-time assertion error.

    Here are important (I think) bits of the code
    Code:
    // function that uses the struct:
    int MakeArincWordAnalog(DataLine *Line, float value,  int word)
    {
    	int  number;
    
    	word =0;
    
    	FILE *out;
        out = fopen("Victor - test.txt", "w");
    
    	fprintf(out, ">%s %d %d %d %d %d %d %d\n<", Line->Param, Line->Label,
                        Line->SFactor, Line->SigBits, Line->Bit30,
    					Line->Bit31, Line->Max, Line->Min);
    
    // The function call in main:
    MakeArincWordAnalog(&FAD.vectDL[i], mtxVicVals[j][i], TempWord);
    
    // the struct and class declarations:
    struct DataLine
    {
    	char Param[20];
    	int Label, SFactor, SigBits;
    	int Bit30, Bit31, Max, Min;
    
    	DataLine()
    	{
    		Label=SFactor=SigBits=Bit30=Bit31=Max=Min=0;
    		memset(Param, 0, sizeof(Param));
    	};
    
    } ;
    
    class FSAD        //Fixed/Selectable Analog Data  
    	{
    	public:
    
    		char Name[25];
    		int nExpected;  // size = nExected*2+1
    		std::vector < DataLine > vectDL;		
    	};
    The code doesn't seem to like the way I pass it the struct.
    Last edited by earth_angel; 07-12-2005 at 02:08 PM.

  2. #2
    Registered User major_small's Avatar
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    well, from what I see, you have DataLine defined under main, and the function defined above main... this is a problem... you need to at least put a prototype of the struct above where you defined the function so the function knows what to expect...

    I haven't looked too far into it, but try that and see if it works...
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  3. #3
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    That code looks fine from the vector perspective. Are you sure i is a valid index to the vector? Also, why are you passing a pointer? Just pass it by value.

  4. #4
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    I actually screwed up the way I worked with my files.
    The rest of the def's was fine.
    here's the whole function:
    Code:
    int MakeWord(FILE *out, DataLine *Line, float value,  int word)
    {
    	int  number;
    
    	word =0;
    
    	/*fprintf(out, ">%s %d %d %d %d %d %d %d\n<", Line->Param, Line->Label,
                        Line->SFactor, Line->SigBits, Line->Bit30,
    					Line->Bit31, Line->Max, Line->Min);*/
    
    	// check if the value is valid
    	if(value == -9999)
    		return 0;
    
    	//Input the label
    	word = MakeLabel(Line->Label);
    	fprintf(out, " label %08x \n", word);
    
    	//Check if value is negative
    	if(value < 0)
    	{
    		word = word | 0x10000000; // Set the sign bit to 1
    		value = -value ;
    	}
    
    	// Set bits 30 
    	if (Line->Bit30 == 1)
    		word = word | 0x20000000;  
    	else
    	{
    		word = word << 3;   
    		word = word >> 3;
    	}
    
    	//and bit 31
    	if (Line->Bit31 ==1)
    		word = word | 0x40000000;
    	else
    	{
    		word = word << 2;
    		word = word >> 2;
    	}
    
    	fprintf(out, " bits 30/31 %08x \n", word);
    
    	// Data part
    	if (Line->SFactor>0)
    	{
    		value = value*(float)pow(2,31)/Line->SFactor;
    		//printf("Value: %08x\n\n", value); 
    		number = ((int)value)>>3;  
    		//printf("Number: %08x\n\n", number);
    	}
    
    	word = word | (number & DATAMASK18);
    
    	fprintf(out, " data %08x \n", word);
    
    	// Set parity to 0
    	word = word << 1;
    	word = word >> 1;
    
    	fprintf(out, " parity %08x \n", word);
    
    	return 1;
    	
    	
    }
    and it's called like this:
    Code:
    MakeWord(out, &FAD.vectDL[i],
    				Value, TempWord);
    when I try to print TempWord after the function call it's empty, but inside the function word is adjusted and the last fprintf states the correct result. Why doesn't it reflect the correct value for TempWord?

  5. #5
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    You are passing TempWord by value. If you want it to be updated when it is modified inside the function, the function should take a reference (for example, int& word). You could also have it take a pointer, but that would require more changes to the function code and the calling code.

  6. #6
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    Yep, I arrived at the same conclussion. So that part is fixed now.
    As always a new problem rose: I have two return statements: One toreturn 0 if the ntry is invalid and the other return 1 if it's all good. I want to use it to check whether I want it to use the value produced by the function or not. For some reason I get a run-time error saying that the memory could not be read.

    Here's the if statement:
    Code:
    if(MakeWord(out, &FAD.vectDL[i], mtxVicVals[j][i], TempWord) == 1)
    and here's the new function declaration with the TempValue reference:
    Code:
    int MakeWord(FILE *out, DataLine *Line, float value,  int &word)

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