std::unique_ptr has an explicit conversion to bool operator. However, according to Wikipedia:
http://en.wikipedia.org/wiki/C%2B%2B0x#Explicit_conversion_operators In C++0x, the explicit keyword can now be applied to conversion operators. As with constructors, it prevents the use of those conversion functions in implicit conversions. However, language contexts that specifically require a boolean value (the conditions of if-statements and loops, as well as operands to the logical operators) count as explicit conversions and can thus use a bool conversion operator. But the following code doen't compile: #include <stdio.h> #include <memory> int main(int argc, char *argv[]) { std::unique_ptr<int> p(0); if (p) {} return 0; } Results in: $ gcc -std=gnu++0x testcase.cpp testcase.cpp: In function 'int main(int, char**)': testcase.cpp:10: error: could not convert 'p' to 'bool' -- Summary: [C++0x] Context-specific explicit conversion doesn't work Product: gcc Version: 4.5.0 Status: UNCONFIRMED Severity: normal Priority: P3 Component: c++ AssignedTo: unassigned at gcc dot gnu dot org ReportedBy: piotr dot wyderski at gmail dot com GCC host triplet: Cygwin/GCC-trunk rev. 147886 http://gcc.gnu.org/bugzilla/show_bug.cgi?id=40283