[C++] Temporary strings implicitly constructed take l-value path instead of r-value path

[C++] Temporary strings implicitly constructed take l-value path instead of r-value path by CodingCat

Closed

as Fixed

Type:	Bug
ID:	669549
Opened:	5/20/2011 1:23:42 AM
Access Restriction:	Public

Workaround(s)

view

User(s) can reproduce this bug

Implicitly constructing temporary string objects by passing C-strings to methods or functions that are overloaded for both r-val references and const l-val references (such as any container insertion method) results in calling of the const-l-val-version rather than the r-val-version, even though the string object passed is evidently only a temporary object.

The problem affects more than just strings, but it is particularly annoying for these, as passing C-strings to methods taking string objects is rather common.
In fact, it seems that any referencing of objects existing outside the expression to be implicitly converted inside this expression provokes this issue, e.g. by implicitly calling int_test_class(int i) with a non-temporary integer.

Details (expand)

Visual Studio 2010 SP1

#include <iostream>
#include <string>

struct int_test_class
{
int i;
int_test_class(int i) : i(i) { }
};
void int_test(const int_test_class &o)
{
std::cout << "int_test:cref; ";
}
void int_test(int_test_class &&o)
{
std::cout << "int_test:rvref; ";
}

struct char_test_class
{
const char *c;
char_test_class(const char *c) : c(c) { }
};
void char_test(const char_test_class &o)
{
std::cout << "char_test:cref; ";
}
void char_test(char_test_class &&o)
{
std::cout << "char_test:rvref; ";
}

void string_test(const std::string &o)
{
std::cout << "string_test:cref; ";
}
void string_test(std::string &&o)
{
std::cout << "string_test:rvref; ";
}

int main()
{
std::cout << "Implicit: ";
int_test(5);
char_test("a");
string_test("a");
std::cout << std::endl;

std::cout << "Explicit: ";
int_test(int_test_class(5));
char_test(char_test_class("a"));
string_test(std::string("a"));
std::cout << std::endl;

return 0;
}

English

Windows Vista

German

Implicit: int_test:rvref; char_test:cref; string_test:cref;
Explicit: int_test:rvref; char_test:rvref; string_test:rvref;

Implicit: int_test:rvref; char_test:rvref; string_test:rvref;
Explicit: int_test:rvref; char_test:rvref; string_test:rvref; (GCC yields this)

0 attachments

Hi,

Thanks for reporting this bug. We've already fixed it, and the fix will be available in VC11. Surprisingly, VC10's behavior was mandated by the C++0x Working Paper at the time (what I refer to as "rvalue references v2.0"). This was fixed by the Standardization Committee (producing what I refer to as "rvalue references v2.1"), and we've partially implemented that in VC11. Our current implementation is enough to fix your test case. Other cases remain to be fixed, and are already tracked by an internal bug.

I talked about this very issue in this video: http://channel9.msdn.com/Shows/Going+Deep/C9-Lectures-Stephan-T-Lavavej-Advanced-STL-4-of-n

If you have any further questions, feel free to E-mail me at stl@microsoft.com .

Stephan T. Lavavej
Visual C++ Libraries Developer