Package: g++-4.0
Version: 4.0.2-5
Severity: normal
Hi,
Given the following (stripped down) code as variant.cpp:
#include <iostream>
using std::cout;
using std::endl;
struct Variant
{
Variant() {}
template< typename T > Variant( T ) {}
Variant( const Variant& ) {}
Variant &operator=( const Variant& ) { return *this; }
template< typename T > operator T() const
{
cout << "operator T() called" << endl;
return T();
}
template< typename T > operator const T&() const
{ static T test_dummy;
cout << "operator T&() called with T = Variant, uh oh" << endl;
return test_dummy;
}
#ifdef WITH_WARNING
operator Variant() const
{
cout << "operator Variant() called" << endl;
return *this;
}
operator const Variant&() const
{
cout << "but operator Variant&() _is_ called" << endl;
return *this;
}
#endif
};
Variant TempMaker( Variant v ) { return v; }
int main()
{
Variant v = TempMaker( 1 );
return 0;
}
The following may be observed:
$ make variant "CPPFLAGS=-Wconversion -DWITH_WARNING"
g++ -Wconversion -DWITH_WARNING variant.cpp -o variant
variant.cpp:39: warning: conversion to the same type will never use a
type conversion operator
variant.cpp:44: warning: conversion to a reference to the same type
will never use a type conversion operator
$ ./variant
but operator Variant&() _is_ called
$ make variant CPPFLAGS=-Wconversion
g++ -Wconversion variant.cpp -o variant
$ ./variant
operator T&() called with T = Variant, uh oh
Indicating that the explicit operators are in fact called, despite
the warning, and are in fact necessary to avoid the compiler attempting
to use the template conversion operator with T = Variant(&) (and doing
bad recursive things in less stripped code) when temporaries are created
for the function call.
The simple answer to me is get rid of the warning, the code operates
intuitively correctly now, but things are rarely simple...
cheers,
Ron
-- System Information:
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Architecture: i386 (i686)
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Kernel: Linux 2.6.12
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Versions of packages g++-4.0 depends on:
ii gcc-4.0 4.0.2-5 The GNU C compiler
ii gcc-4.0-base 4.0.2-5 The GNU Compiler Collection (base
ii libc6 2.3.5-9 GNU C Library: Shared libraries an
ii libstdc++6-4.0-dev 4.0.2-5 The GNU Standard C++ Library v3 (d
g++-4.0 recommends no packages.
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