Add non-const version of value::at()

[Yosh31207]

value::at() returns value with const qualifier even if the value is non-const data.

Version of Boost

Boost 1.79

Example

value jv1 = {{"key", 1}};
auto& a1  = jv1.as_object().at("key");  // returns value&
auto& a2  = jv1.at("key");              // returns const value&
value& a3 = jv1.at("key");              // compile error

value jv2 = {1, 2, 3};
auto& b1  = jv2.as_array().at(0);       // returns value&
auto& b2  = jv2.at(0);                  // returns const value&
value& b3 = jv2.at(0);                  // compile error

Note: value.at_pointer() has no such issue. It returns const value& if the data is const-qualified and returns value& otherwise.

value jv1 = {{"key", 1}};
auto& a1  = jv1.as_object().at("key");  // returns value&
auto& a2  = jv1.at_pointer("/key");     // returns value&

const value jv2 = {{"key", 1}};
auto& b1  = jv1.as_object.at("key");    // returns const value&
auto& b2  = jv1.at_pointer("/key");     // returns const value&