116. bitset cannot be constructed with a const char*

Section: 22.9.2 [template.bitset] Status: Dup Submitter: Judy Ward Opened: 1998-11-06 Last modified: 2016-01-28

Priority: Not Prioritized

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Duplicate of: 778

Discussion:

The following code does not compile with the EDG compiler:

#include <bitset>
using namespace std;
bitset<32> b("111111111");

If you cast the ctor argument to a string, i.e.:

bitset<32> b(string("111111111"));

then it will compile. The reason is that bitset has the following templatized constructor:

template <class charT, class traits, class Allocator>
explicit bitset (const basic_string<charT, traits, Allocator>& str, ...);

According to the compiler vendor, Steve Adamcyk at EDG, the user cannot pass this template constructor a const char* and expect a conversion to basic_string. The reason is "When you have a template constructor, it can get used in contexts where type deduction can be done. Type deduction basically comes up with exact matches, not ones involving conversions."

I don't think the intention when this constructor became templatized was for construction from a const char* to no longer work.

Proposed resolution:

Add to 22.9.2 [template.bitset] a bitset constructor declaration

explicit bitset(const char*);

and in Section 22.9.2.2 [bitset.cons] add:

explicit bitset(const char* str);

Effects:
    Calls bitset((string) str, 0, string::npos);

Rationale:

Although the problem is real, the standard is designed that way so it is not a defect. Education is the immediate workaround. A future standard may wish to consider the Proposed Resolution as an extension.