在匿名实例创建中使用先前变量时发生重新声明错误

Question

g++ gives a re-declaration error when using previously declared variable in anonymous instance creation.

I have the following code in my "weird.cpp" source file:

#include <iostream>

int main()
{
  int i = 0;
  int j = int ( i );
  int ( i );
}

The error i am getting is,

weird.cpp: In function ‘int main()’:
weird.cpp:7: error: redeclaration of ‘int i’
weird.cpp:5: error: ‘int i’ previously declared here

I have tried this in mac and linux with versions 4.2 and 4.7 respectively. I have also tried with other types instead of int. The result is the same error. Can anyone help me understand this problem? Thanks.

Answer 1

First of all, the parentheses you're using here don't do anything.

int i = 0;
int j = int(i); // This is casting i to an int. It's already an int.
int j = i; // This does the same as the last line.
int (i); // This is redeclaring an int named i, which had already been done.
int i; // This is the same as the last line.

What you are saying about an object accepting an int in it's constructor doesn't make sense.

struct A { A(int) {} };
int i;
A obj(i); // A instance named obj which takes integer i as constructor argument.

I don't really understand what you're trying to achieve here, perhaps this?

int i = 0;
int j = i;
{
    int i; // In another scope, shadowing the first i for scope duration.
}

Answer 2

You could be forgiven for being confused by this, it's a case of C++'s context-sensitive nature and how that is interpreted by the compiler.

int (i);

is being treated as a declaration of "i" (and since you already have a variable called i in this scope and have not enabled -Wno-shadow, it's not allowing this).

Contrast with the following case, which doesn't compile: (see http://ideone.com/QuwnTC )

#include <iostream>

class Bark {
public:
    Bark(const char* msg, const char*) {
         std::cout << "Hear ye, hear ye. " << msg << std::endl;
    }
};

void bark(const char* i) {
    Bark (i); // error here.
}

int main(int argc, const char* argv) {
    bark("wtf");
}

It complains that Bark (i) shadows "i"s declaration.

However, both of the following DO compile: http://ideone.com/dcGMET

void bark(const char* i) {
    Bark (i + 1);
}

or having two arguments inside the parenthesis: ( http://ideone.com/tMzSY9 )

#include <iostream>

class Bark {
public:
    Bark(const char* msg, const char*) {
         std::cout << "Hear ye, hear ye. " << msg << std::endl;
    }
};

void bark(const char* i) {
    Bark (i, NULL);
}

int main(int argc, const char* argv) {
    bark("wtf");
}

Clearly, the treatment of "type (name)" here is some sort of special case, and you might want to raise this with the compiler developers.