Question

Is there a better way to initialise C structures in C++ code?

I can use initialiser lists at the variable declaration point; however, this isn’t that useful if all arguments are not known at compile time, or if I’m not declaring a local/global instance, eg:

Legacy C code which declares the struct, and also has API’s using it

typedef struct
{
    int x, y, z;
} MyStruct;

C++ code using the C library

void doSomething(std::vector<MyStruct> &items)
{
    items.push_back(MyStruct(5,rand()%100,items.size()));//doesn't work because there is no such constructor
    items.push_back({5,rand()%100,items.size()});//not allowed either

    //works, but much more to write...
    MyStruct v;
    v.x = 5;
    v.y = rand()%100;
    v.z = items.size();
    items.push_back(v);
}

Creating local instances and then setting each member one at a time (myStruct.x = 5; etc) is a real pain, and somewhat hard to read when trying to add say 20 different items to the container…

Answer 1

If you can’t add a constructor (which is the best solution in C++03 but you probably have compatibility constraint with C), you can write a function with the same effect:

MyStruct makeAMyStruct(int x, int y, int z)
{
    MyStruct result = { x, y, z };
    return result;
}

items.push_back(makeAMyStruct(5,rand()%100,items.size()));

Edit: I’d have checked now that C++0X offers something for this precise problem:

items.push_back(MyStruct{5,rand()%100,items.size()});

which is available in g++ 4.4.