Filter the types that get fallback lexical conversion operators

Only enable the operators for types that already have iostream formatting
operators defined.  Having them exist for everything would interfere with
some other stuff I'm going to commit in a moment.

diff --git a/source/core/meta.h b/source/core/meta.h
index 0034512b6ec2bea573ffbcc34115fc8562a5cac8..923096517dba8132a873de68a79344ab6e2910b1 100644 (file)
--- a/source/core/meta.h
+++ b/source/core/meta.h
@@ -11,6 +11,7 @@ template<typename T>
 struct RemoveConst<const T>
 { typedef T Type; };
 
+
 template<typename T>
 struct RemoveReference
 { typedef T Type; };
@@ -19,6 +20,34 @@ template<typename T>
 struct RemoveReference<T &>
 { typedef T Type; };
 
+
+template<bool c, typename R>
+struct EnableIf;
+
+template<typename R>
+struct EnableIf<true, R>
+{ typedef R Yes; };
+
+template<typename R>
+struct EnableIf<false, R>
+{ typedef R No; };
+
+
+/**
+Common fragments used in SFINAE-based decider constructs.
+*/
+struct Sfinae
+{
+       struct Yes { char c[2]; };
+       struct No { char c; };
+
+       template<size_t s>
+       struct Evaluate
+       {
+               enum { value = (s==sizeof(Yes)) };
+       };
+};
+
 } // namespace Msp
 
 #endif

diff --git a/source/strings/lexicalcast.h b/source/strings/lexicalcast.h
index c6a17da3491d053b6460399fddea6473ea1ca839..80963f44dd3a233ff2d16b01ec007935d01e73c9 100644 (file)
--- a/source/strings/lexicalcast.h
+++ b/source/strings/lexicalcast.h
@@ -4,6 +4,7 @@
 #include <sstream>
 #include <string>
 #include <stdexcept>
+#include <msp/core/meta.h>
 #include "fmt.h"
 
 namespace Msp {
@@ -89,7 +90,39 @@ void operator>>(const LexicalConverter &, std::string &);
 // Generic operators using stringstream
 
 template<typename T>
-void operator<<(LexicalConverter &c, const T &v)
+struct HasFormattedOutput: Sfinae
+{
+       static std::ostream &s;
+       static T &v;
+
+       /* The expression must depend on the template parameter, or the compiler
+       will give an error. */
+       template<typename U>
+       static Yes f(int (*)[sizeof(s<<HasFormattedOutput<U>::v)]);
+       template<typename U>
+       static No f(...);
+
+       enum { value = Evaluate<sizeof(f<T>(0))>::value };
+};
+
+template<typename T>
+struct HasFormattedInput: Sfinae
+{
+       static std::istream &s;
+       static T &v;
+
+       template<typename U>
+       static Yes f(int (*)[sizeof(s>>HasFormattedOutput<U>::v)]);
+       template<typename U>
+       static No f(...);
+
+       enum { value = Evaluate<sizeof(f<T>(0))>::value };
+};
+
+
+template<typename T>
+typename EnableIf<HasFormattedOutput<T>::value, void>::Yes
+operator<<(LexicalConverter &c, const T &v)
 {
        std::ostringstream ss;
        ss<<c.get_fmt()<<v;
@@ -97,7 +130,8 @@ void operator<<(LexicalConverter &c, const T &v)
 }
 
 template<typename T>
-void operator>>(const LexicalConverter &c, T &v)
+typename EnableIf<HasFormattedInput<T>::value, void>::Yes
+operator>>(const LexicalConverter &c, T &v)
 {
        std::istringstream ss(c.get());
        ss.setf(std::ios_base::fmtflags(0), std::ios_base::skipws);