MySet(20,100) = "abcd"; copy and/or assignment?

Question

apologies if this answer should be obvious, perhaps this has a pattern name like "dopey assignment overload" and have not been searching by right topic - redirects most welcome code extracts:

class MySet {
        private:
                int lower;
                int upper;
                char *data;
        public:
                MySet( int l, int u, const char *val = 0 );

                MySet &operator=( MySet &rhs );
                MySet &operator=( const char *val );
};

int main(int i, const char *argv[])
{
        MySet A(1,10);
        A = "hello";

        MySet B(1,10, "hello");
        MySet C(1,10) = "hello";

}

assignments for A and B work but for C then g++ whines....: myset.cxx:19:16: error: expected ‘,’ or ‘;’ before ‘=’ token

is not a big issue but have been trying to understand why is not valid.

thanks for any helpful replies.

more info... guess am meant to edit my op?

on puzzling out why the above is considered illegal, have been looking for a definitive reason ... and of course a trick to make such notation be accepted by the compiler.

considering the similar uses that are legal

struct toons {
     char first[10];
} flintstone[] = {
     "fred",
     "wilma",
     ""
};

works from couple decades of C compilers, as does:

typedef struct {
    char family[10];
    struct {
            char name[10];
    } members[6];
} toongroup;

toongroup rubble = {
    "rubble",
    {
            "barney",
            "wilma",
            ""
    }
}

toongroup rubble = {
    "rubble",
    {
            "barney",
            "wilma",
            "", "", "", ""
    }
};

toongroup jetsons = { "jetson", { "george", "jane", "", "", "", "" } };

all honored by c++ compilers. can have member functions on the structure and it still works:

typedef struct toongroup {
    public:
            bool add( const char *newbaby ) {
                    for(int i; i<6; i++) {
                            if(strlen(members[i].name) == 0) {
                                    strcpy(members[i].name, newbaby);
                                    return true;
                            }
                            if(strcmp(members[i].name, newbaby) == 0)
                                    return false;
                    }
                    return false;
            }
    char family[10];
    struct {
            char name[10];
    } members[6];
} toongroup;

toongroup jetsons = { "jetson", { "george", "jane", "", "", "", "" } };

jetsons.add( "elroy" );

objections start when adding a constructor:

typedef struct toongroup {
    toongroup( const char *f, const char *m, ... ) {
    }


error: in C++98 ‘rubble’ must be initialized by constructor, not by ‘{...}’
error: could not convert ‘{"rubble", {"barney", "wilma", "", "", "", ""}}’ from ‘<brace-enclosed initializer list>’ to ‘toongroup‘

wondering if it could be a matter of using the right initializer so compiler can match the constructor

    toongroup empty;
    toongroup nobody = empty;
    toongroup rubble( "rubble", "barney", "wilma", "" );

even

    toongroup empty;
    toongroup cancelled;
    toongroup nobody = empty;
    toongroup rubble( "rubble", "barney", "wilma", "" );
    toongroup jetsons( "jetson", "george", "jane", "" );

    jetsons.add( "elroy" );

adding

    toongroup &operator=( toons &t );

allows

    jetsons = cancelled;

but not

    toongroup jetsons( "jetson", "george", "jane", "" ) = cancelled;

apologies for this ending up a long post, but am looking for some reference as to why the constructor + assignment is rejected and any ideas on how/if this can be accommodated.

in the notation for MySet this would be a familiar statement, before i declared it illegal wanted to check if am missing a trick.

thanks.

Answer 1

Because

MySet C(1,10) = "hello";

is illegal. You can't call assignment or anything else when you initialize a variable.

If it did work, the = would stand for initialization, not assignment.

Just like:

int x = 5;  //initialization, not assignment

is equivalent to

int x(5);

and not

int x;
x = 5;

Answer 2

The problem is that when you write:

MySet C = "hello";

Actually the constructor from character array is invoked (if such existed), not the assignment operator. Thus with your code you confuse the compiler: which of two constructors to call.

Basically equality on the declaration row is converted to constructor call. From then on the assignment operators are used.

Answer 3

Remember 2 points:

1. When a variable is declared, the subsequent = is not an assignment operator, but rather meant for construction of the object.
2. C++ compiler allows only 1 constructor at a time.

Keeping those in mind, look at the problematic code:

MySet C(1,10) = "hello";

This declares and object of MySet and constructs it with (1,10).
Now there is no point of having another construction using = "hello" which would overpower the existing construction.
That's why the error.