C++ how to pass values from one structure to another?

I have two different structures. The first, astm, which reads data from a .txt file, and the second, rebar, uses the values from structure one to read user input, and make calculations. Sturct astm will act as a menu. My question is, how do I pass the values from struct astm to struct rebar?

here are the two structures:

typedef struct
{
    int size; 
    double weight;
    double diameter;
 } astm;

 typedef struct
 {
    int size;// user input from astm.size
    double length; // user input
  } rebar;

here is the .txt file ( actual file doesn't contain "sz", "wt", or "diameter"):

sz   wt    diameter
2   0.167  0.250
3   0.376  0.375
4   0.668  0.500
5   1.043  0.625
6   1.502  0.750
7   2.044  0.875
8   2.670  1.000
9   3.400  1.128
10  4.303  1.27
11  5.313  1.41
14  7.65   1.693
1开发者_开发百科8  13.6   2.257

Example: Select a size: 4 Enther length: 500

Size: 4 diameter: 0.500 Total weight = length * weight = 3340.0

---

To answer your question:

astm myAstm;
myAstm.size = 5;
myAstm.weight = 30.0;
myAstm.diameter = 10;

rebar myRebar;
myRebar.size = myAstm.size;
myRebar.length = someCalculation(...)

However, a better approach would be to allow one to be embedded into the other:

typedef struct
{
    int size; 
    double weight;
    double diameter;
} astm;

typedef struct
{
    astm *myAstm;
    double length; // user input
} rebar;

Doing it this way would result in less redundant data lying around, and it would allow you to pass around exactly one structure which represents everything.

astm myAstm;
myAstm.size = 5;
myAstm.weight = 30.0;
myAstm.diameter = 10;

rebar myRebar;
myRebar.myAstm = &myAstm;
myRebar.length = someCalculation(...)

// To get at the size attribute:
printf("size is %d\n", myRebar.myAstm->size);

---

You could overload the assignment operator.

But be careful, operator overloading is not always the best way.

Maybe, a convert function, or a member function is the better choice.

---

Overload the assignment operator is one possible route, but not many are not fond of this. You could also pass them directly to the member variable.

objRebar.size = objastm.size;

Or you could make these into classes and add a member function to handle it for you. Is it necessary that you use a struct?

---

Ideally, you would like to have a map that looks up the weight / diameter based on the size. Since you are using C++, take a look at std::map ( http://www.cplusplus.com/reference/stl/map/ ).

change astm to hold only double weight, diameter; and when reading the txt file for each sz, do a map.insert(pair<int,astm>(sz,astm_var));

When calculating, simply lookup the astm from the map and evaluate the total weight from it.

---

I would solve this by creating a function:

void MoveInfo(const Astm &t, ReBar &bar);

But to do that you would need to have the struct names provided properly:

struct Astm { ... };
struct ReBar { ... };

Notice the position of the struct name.

---

If rebar was using astm directly (and you're actually using C++), you would have something like this:

struct astm
{
    astm( int size, float weight, float diameter )
    : size( size )
    , weight( weight )
    , diameter( diameter )
    {}

    int size;
    double weight;
    double diameter;
};

struct rebar
{
    rebar( int size, double length )
    : size( size )
    , length( length )
    {}

    rebar( const astm& astm ) //<<< Uses astm directly
    : size( astm.size )
    {
        // Do something with rest of astm
    }
};

However, this does not appear to be the case. Sounds like you want something like:

    std::vector<rebar> rebarVec;
    for ( int i = 0; i < numThings; ++i )
    {
        // Compute stuff from astm[ i ]
        rebar rebarItem( size, length );
        rebarVec.push_back( rebarItem );
    }

Is this what you're trying to accomplish?