CODEWITHSUNDEEP
c++constructor
TheBat
TheBat

Reputation: 1054

C++ Ambiguity of default constructor and something else. What is that something else?

I'm not sure what search terms I even need to use to find the answer to this question, so sorry if this is duplicated. Basically, Why does this compile?

struct A {                                                                                                                                                                                                                                                                                                                   
    A(){};                                                                                                                                                                                                                                                                                                                   
    A(const char*){};                                                                                                                                                                                                                                                                                                        
};                                                                                                                                                                                                                                                                                                                           

int main() {                                                                                                                                                                                                                                                                                                                 
    //const char* b = "what is going on?";                                                                                                                                                                                                                                                                                                  
    A(b);                                                                                                                                                                                                                                                                                                                    
    return 0;                                                                                                                                                                                                                                                                                                                
}

But this does not?

struct A {                                                                                                                                                                                                                                                                                                                   
    A(){};                                                                                                                                                                                                                                                                                                                   
    A(const char*){};                                                                                                                                                                                                                                                                                                        
};                                                                                                                                                                                                                                                                                                                           

int main() {                                                                                                                                                                                                                                                                                                                 
    const char* b = "what is going on?";                                                                                                                                                                                                                                                                                                  
    A(b);                                                                                                                                                                                                                                                                                                                    
    return 0;                                                                                                                                                                                                                                                                                                                
}

test.cpp: In function ‘int main()’:
test.cpp:8: error: conflicting declaration ‘A b’
test.cpp:7: error: ‘b’ has a previous declaration as ‘const char* b’

What feature of c++ is causing this ambiguity? The main goal of this is to create an anonymous variable on the stack of type A. Similar to A a(b);, but not naming a.

Upvotes: 2

Views: 111

Answers (1)

iz_
iz_

Reputation: 16633

This is caused by an ambiguity in the C++ grammar. A(b); is parsed as a definition of b, an object of type A. This exact issue is described in [stmt.ambig].

To fix this, either use uniform initialization A{b}; or surround it with parentheses to force it to be an expression instead of a declaration (A(b));. Either fix will allow your program to compile.

Upvotes: 5

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