work on #144

README.md

@@ -398,7 +398,7 @@ $ make
 $ ./json_unit "*"
 
 ===============================================================================
-All tests passed (3341846 assertions in 28 test cases)
+All tests passed (3341848 assertions in 28 test cases)
 ```
 
 For more information, have a look at the file [.travis.yml](https://github.com/nlohmann/json/blob/master/.travis.yml).

src/json.hpp

@@ -2550,7 +2550,9 @@ class basic_json
 
     @tparam ValueType non-pointer type compatible to the JSON value, for
     instance `int` for JSON integer numbers, `bool` for JSON booleans, or
-    `std::vector` types for JSON arrays
+    `std::vector` types for JSON arrays. The character type of @ref string_t
+    as well as an initializer list of this type is excluded to avoid
+    ambiguities as these types implicitly convert to `std::string`.
 
     @return copy of the JSON value, converted to type @a ValueType
 
@@ -2571,6 +2573,8 @@ class basic_json
     template<typename ValueType, typename
              std::enable_if<
                  not std::is_pointer<ValueType>::value
+                 and not std::is_same<ValueType, typename string_t::value_type>::value
+                 and not std::is_same<ValueType, std::initializer_list<typename string_t::value_type>>::value
                  , int>::type = 0>
     operator ValueType() const
     {

src/json.hpp.re2c

@@ -2550,7 +2550,9 @@ class basic_json
 
     @tparam ValueType non-pointer type compatible to the JSON value, for
     instance `int` for JSON integer numbers, `bool` for JSON booleans, or
-    `std::vector` types for JSON arrays
+    `std::vector` types for JSON arrays. The character type of @ref string_t
+    as well as an initializer list of this type is excluded to avoid
+    ambiguities as these types implicitly convert to `std::string`.
 
     @return copy of the JSON value, converted to type @a ValueType
 
@@ -2571,6 +2573,8 @@ class basic_json
     template<typename ValueType, typename
              std::enable_if<
                  not std::is_pointer<ValueType>::value
+                 and not std::is_same<ValueType, typename string_t::value_type>::value
+                 and not std::is_same<ValueType, std::initializer_list<typename string_t::value_type>>::value
                  , int>::type = 0>
     operator ValueType() const
     {

test/unit.cpp

@@ -10323,6 +10323,19 @@ TEST_CASE("regression tests")
         CHECK(j["string"] == "\u0007\u0007");
     }
 
+    SECTION("issue #144 - implicit assignment to std::string fails")
+    {
+        json o = {{"name", "value"}};
+
+        std::string s1 = o["name"];
+        CHECK(s1 == "value");
+
+        std::string s2;
+        s2 = o["name"];
+
+        CHECK(s2 == "value");
+    }
+
     SECTION("character following a surrogate pair is skipped")
     {
         CHECK(json::parse("\"\\ud80c\\udc60abc\"").get<json::string_t>() == u8"\U00013060abc");