how to define a var in C++

Question

This code compiles perfect:

if ( args.Length() > 0 ) {
    if ( args[0]->IsString() ) {
        String::Utf8Value szQMN( args[0]->ToString() ) ;
        printf( "(cc)>>>> qmn is [%s].\n", (const char*)(* szQMN) ) ;
    } ;
} ;

But this one does not :

if ( args.Length() > 0 ) {
    if ( args[0]->IsString() ) {
        String::Utf8Value szQMN( args[0]->ToString() ) ; // <<<< (A)
    } ;
} ;
printf( "(cc)>>>> qmn is [%s].\n", (const char*)(* szQMN) ) ; // <<<< (B)

Error says : "error C2065: 'szQMN' : undeclared identifier" on line (B)

This means to me that the sentence marked (A) is a definition at the same time as an assignement, right ?

And compiler decides it is "conditionally" defined as it is within two "IF's" ?

My question is : how to move the declaration out of the two "IF's" ?

In this way I also can give it a defalut value ... in case a IF fails.

If I write this line out of the two "IF's"

String::Utf8Value szQMN ("") ;

... then I get the error :

cannot convert argument 1 from 'const char [1]' to 'v8::Handle<v8::Value>'

Any ideas?

Answer 1

This means to me that the sentence marked (A) is a definition at the same time as an assignement, right?

Technically it is a constructor call that creates a variable and initializes it.

Also note that automatic variables exist only until the end of the scope (usually a block inside {} brackets). That is why your second code example does not compile.

if (condition)
{
    int x = 5;
}
x = 6; // error, x does not exist anymore

My question is : how to move the declaration out of the two "IF's"?

String::Utf8Value szQMN ("");

This is a constructor call of the class String::Utf8Value class. From the error message it takes a parameter of type v8::Handle<v8::Value>. Without knowing what this is I cannot give you an answer how to call it. You wanted to pass "" which is of type const char* or const char[1] and the compiler is telling you that it does not take that parameter.

Edit:

From the link that DeepBlackDwarf provided in the comment, this is how you create a Utf8Value from a string:

std::string something("hello world"); 
Handle<Value> something_else = String::New( something.c_str() );

So in your case you would do:

String::Utf8Value szQMN (String::New(""));

Answer 2

the definition in the IF's loop is only works in this loop. So in the (B) sentence,the definition has already been expired. If you need to use the var both in the IF's loop and outside,you can declare a global variability by this sentence :

extern String::Utf8Value szQMN( args[0]->ToString() ) ;