improved documentation for #289

README.md

@@ -498,7 +498,7 @@ Thanks a lot for helping out!
 
 ## Notes
 
-- The code contains numerous debug **assertions** which can be switched off by defining the preprocessor macro `NDEBUG`, see the [documentation of `assert`](http://en.cppreference.com/w/cpp/error/assert).
+- The code contains numerous debug **assertions** which can be switched off by defining the preprocessor macro `NDEBUG`, see the [documentation of `assert`](http://en.cppreference.com/w/cpp/error/assert). In particular, note [`operator[]`](https://nlohmann.github.io/json/classnlohmann_1_1basic__json_a2e26bd0b0168abb61f67ad5bcd5b9fa1.html#a2e26bd0b0168abb61f67ad5bcd5b9fa1) implements **unchecked access** for const objects: If the given key is not present, the behavior is undefined (think of a dereferenced null pointer) and yields an [assertion failure](https://github.com/nlohmann/json/issues/289) if assertions are switched on. If you are not sure whether an element in an object exists, use checked access with the [`at()` function](https://nlohmann.github.io/json/classnlohmann_1_1basic__json_a674de1ee73e6bf4843fc5dc1351fb726.html#a674de1ee73e6bf4843fc5dc1351fb726).
 - As the exact type of a number is not defined in the [JSON specification](http://rfc7159.net/rfc7159), this library tries to choose the best fitting C++ number type automatically. As a result, the type `double` may be used to store numbers which may yield [**floating-point exceptions**](https://github.com/nlohmann/json/issues/181) in certain rare situations if floating-point exceptions have been unmasked in the calling code. These exceptions are not caused by the library and need to be fixed in the calling code, such as by re-masking the exceptions prior to calling library functions.
 
 

src/json.hpp

@@ -1843,7 +1843,8 @@ class basic_json
     @param[in] first begin of the range to copy from (included)
     @param[in] last end of the range to copy from (excluded)
 
-    @pre Iterators @a first and @a last must be initialized.
+    @pre Iterators @a first and @a last must be initialized. **This
+         precondition is enforced with an assertion.**
 
     @throw std::domain_error if iterators are not compatible; that is, do not
     belong to the same JSON value; example: `"iterators are not compatible"`
@@ -3509,6 +3510,9 @@ class basic_json
 
     @return const reference to the element at key @a key
 
+    @pre The element with key @a key must exist. **This precondition is
+         enforced with an assertion.**
+
     @throw std::domain_error if JSON is not an object; example: `"cannot use
     operator[] with null"`
 
@@ -3667,6 +3671,9 @@ class basic_json
 
     @return const reference to the element at key @a key
 
+    @pre The element with key @a key must exist. **This precondition is
+         enforced with an assertion.**
+
     @throw std::domain_error if JSON is not an object; example: `"cannot use
     operator[] with null"`
 
@@ -3867,7 +3874,8 @@ class basic_json
     @complexity Constant.
 
     @pre The JSON value must not be `null` (would throw `std::out_of_range`)
-    or an empty array or object (undefined behavior, guarded by assertions).
+    or an empty array or object (undefined behavior, **guarded by
+    assertions**).
     @post The JSON value remains unchanged.
 
     @throw std::out_of_range when called on `null` value
@@ -3909,7 +3917,8 @@ class basic_json
     @complexity Constant.
 
     @pre The JSON value must not be `null` (would throw `std::out_of_range`)
-    or an empty array or object (undefined behavior, guarded by assertions).
+    or an empty array or object (undefined behavior, **guarded by
+    assertions**).
     @post The JSON value remains unchanged.
 
     @throw std::out_of_range when called on `null` value.
@@ -6592,8 +6601,8 @@ class basic_json
     @note An iterator is called *initialized* when a pointer to a JSON value
           has been set (e.g., by a constructor or a copy assignment). If the
           iterator is default-constructed, it is *uninitialized* and most
-          methods are undefined. The library uses assertions to detect calls
-          on uninitialized iterators.
+          methods are undefined. **The library uses assertions to detect calls
+          on uninitialized iterators.**
 
     @requirement The class satisfies the following concept requirements:
     - [RandomAccessIterator](http://en.cppreference.com/w/cpp/concept/RandomAccessIterator):

src/json.hpp.re2c

@@ -1843,7 +1843,8 @@ class basic_json
     @param[in] first begin of the range to copy from (included)
     @param[in] last end of the range to copy from (excluded)
 
-    @pre Iterators @a first and @a last must be initialized.
+    @pre Iterators @a first and @a last must be initialized. **This
+         precondition is enforced with an assertion.**
 
     @throw std::domain_error if iterators are not compatible; that is, do not
     belong to the same JSON value; example: `"iterators are not compatible"`
@@ -3509,6 +3510,9 @@ class basic_json
 
     @return const reference to the element at key @a key
 
+    @pre The element with key @a key must exist. **This precondition is
+         enforced with an assertion.**
+
     @throw std::domain_error if JSON is not an object; example: `"cannot use
     operator[] with null"`
 
@@ -3667,6 +3671,9 @@ class basic_json
 
     @return const reference to the element at key @a key
 
+    @pre The element with key @a key must exist. **This precondition is
+         enforced with an assertion.**
+
     @throw std::domain_error if JSON is not an object; example: `"cannot use
     operator[] with null"`
 
@@ -3867,7 +3874,8 @@ class basic_json
     @complexity Constant.
 
     @pre The JSON value must not be `null` (would throw `std::out_of_range`)
-    or an empty array or object (undefined behavior, guarded by assertions).
+    or an empty array or object (undefined behavior, **guarded by
+    assertions**).
     @post The JSON value remains unchanged.
 
     @throw std::out_of_range when called on `null` value
@@ -3909,7 +3917,8 @@ class basic_json
     @complexity Constant.
 
     @pre The JSON value must not be `null` (would throw `std::out_of_range`)
-    or an empty array or object (undefined behavior, guarded by assertions).
+    or an empty array or object (undefined behavior, **guarded by
+    assertions**).
     @post The JSON value remains unchanged.
 
     @throw std::out_of_range when called on `null` value.
@@ -6592,8 +6601,8 @@ class basic_json
     @note An iterator is called *initialized* when a pointer to a JSON value
           has been set (e.g., by a constructor or a copy assignment). If the
           iterator is default-constructed, it is *uninitialized* and most
-          methods are undefined. The library uses assertions to detect calls
-          on uninitialized iterators.
+          methods are undefined. **The library uses assertions to detect calls
+          on uninitialized iterators.**
 
     @requirement The class satisfies the following concept requirements:
     - [RandomAccessIterator](http://en.cppreference.com/w/cpp/concept/RandomAccessIterator):