Fix formatted output operator detection in lexicalcast.h

[libs/core.git] / source / strings / lexicalcast.h

diff --git a/source/strings/lexicalcast.h b/source/strings/lexicalcast.h
index b6cba3b89498c32b86576c66087d9472a0a4c680..22b45e88c55eef60e433b6253d0070b77c9242a5 100644 (file)
--- a/source/strings/lexicalcast.h
+++ b/source/strings/lexicalcast.h
@@ -4,12 +4,13 @@
 #include <sstream>
 #include <string>
 #include <stdexcept>
+#include <msp/core/meta.h>
 #include "fmt.h"
 
 namespace Msp {
 
 /**
-Thrown for errors in lexical conversions
+Thrown for errors in lexical conversions.
 */
 class lexical_error: public std::runtime_error
 {
@@ -19,6 +20,9 @@ public:
 };
 
 
+/**
+Thrown when the format is unsuitable for the type being converted.
+*/
 class format_mismatch: public lexical_error
 {
 public:
@@ -34,14 +38,15 @@ class LexicalConverter
 {
 private:
        Fmt fmt;
+       bool filled;
        std::string buf;
 
 public:
-       LexicalConverter(const Fmt &f): fmt(f) { }
-       LexicalConverter(const std::string &s, const Fmt &f): fmt(f), buf(s) { }
+       LexicalConverter(const Fmt &f): fmt(f), filled(false) { }
+       LexicalConverter(const std::string &s, const Fmt &f): fmt(f), filled(true), buf(s) { }
 
        const Fmt &get_fmt() const { return fmt; }
-       const std::string &get() const { return buf; }
+       const std::string &get() const;
        void result(const std::string &);
 };
 
@@ -88,41 +93,94 @@ void operator>>(const LexicalConverter &, std::string &);
 
 // Generic operators using stringstream
 
+struct CheckFormattedOutput: Sfinae
+{
+       static std::ostream &s;
+       template<typename T>
+       static Yes f(int (*)[sizeof(s<<reinterpret_cast<const T &>(s))]);
+       using Sfinae::f;
+};
+
+struct CheckFormattedInput: Sfinae
+{
+       static std::istream &s;
+       template<typename T>
+       static Yes f(int (*)[sizeof(s>>reinterpret_cast<T &>(s))]);
+       using Sfinae::f;
+};
+
+template<typename T> struct HasFormattedOutput: Sfinae::Evaluate<CheckFormattedOutput, T> { };
+template<typename T> struct HasFormattedInput: Sfinae::Evaluate<CheckFormattedInput, T> { };
+
+
 template<typename T>
-void operator<<(LexicalConverter &c, const T &v)
+typename EnableIf<HasFormattedOutput<T>::value, void>::Yes
+operator<<(LexicalConverter &c, const T &v)
 {
        std::ostringstream ss;
-       ss<<c.get_fmt()<<v;
+       ss << c.get_fmt() << v;
        c.result(ss.str());
 }
 
 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);
-       ss>>v;
+       ss >> v;
        if(ss.fail() || !ss.eof())
                throw lexical_error("conversion failure");
 }
 
-// The main interface to the lexical conversion machinery
+/**
+Helper struct to provide partial template specialization.
+*/
+template<typename T, typename F>
+struct LexicalCast;
 
 template<typename T>
-inline T lexical_cast(const std::string &s, const Fmt &f = Fmt())
+struct LexicalCast<T, std::string>
 {
-       LexicalConverter conv(s, f);
-       T result;
-       conv>>result;
-       return result;
-}
+       static T cast(const std::string &s, const Fmt &f = Fmt())
+       {
+               LexicalConverter conv(s, f);
+               T result;
+               conv >> result;
+               return result;
+       }
+};
 
-template<typename T>
-inline std::string lexical_cast(const T &v, const Fmt &f = Fmt())
+template<typename F>
+struct LexicalCast<std::string, F>
+{
+       static std::string cast(const F &v, const Fmt &f = Fmt())
+       {
+               LexicalConverter conv(f);
+               conv << v;
+               return conv.get();
+       }
+};
+
+template<>
+struct LexicalCast<std::string, std::string>
+{
+       static std::string cast(const std::string &v, const Fmt &f = Fmt())
+       {
+               LexicalConverter conv(f);
+               conv << v;
+               return conv.get();
+       }
+};
+
+/** Perform a lexical conversion between a string and another type.  The source
+type can normally be deduced by the compiler, so this can be used just like the
+standard C++ casts.  A format may additionally be specified to force a specific
+interpretation. */
+template<typename T, typename F>
+inline T lexical_cast(const F &v, const Fmt &f = Fmt())
 {
-       LexicalConverter conv(f);
-       conv<<v;
-       return conv.get();
+       return LexicalCast<T, F>::cast(v, f);
 }
 
 } // namespace Msp