Adding to a Protocol Buffers repeated field

Question

I'm working in C++ with a Protocol Buffer template including the following message:

message StringTable {
   repeated bytes s = 1;
}

I'm attempting to add a new value to the existing data, like so:

pb.stringtable().s().Add(replace_key);

However, this generates an error on compilation (using clang on OS X):

test.cpp:51:4: error: member function 'Add' not viable: 'this' argument 
  has type 'const ::google::protobuf::RepeatedPtrField< ::std::string>', 
  but function is not marked const
                    pb.stringtable().s().Add(replace_key);
                    ^~~~~~~~~~~~~~~~~~~~

Any clues? I'm very much a C++ newbie so may be making a dumb error.

---

Edit:

Using the accessors produces a similar error:

pb.stringtable().add_s(replace_key);

results in:

test.cpp:51:21: error: no matching member function for call to 'add_s'
                        pb.stringtable().add_s(replace_key);
                        ~~~~~~~~~~~~~~~~~^~~~~
./osmformat.pb.h:3046:26: note: candidate function not viable: 'this' argument has type 'const ::StringTable', but method is not marked const
inline void StringTable::add_s(const ::std::string& value) {
                     ^
./osmformat.pb.h:3050:26: note: candidate function not viable: 'this' argument has type 'const ::StringTable', but method is not marked const
inline void StringTable::add_s(const char* value) {
                     ^
./osmformat.pb.h:3043:36: note: candidate function not viable: requires 0 arguments, but 1 was provided
inline ::std::string* StringTable::add_s() {
                               ^
./osmformat.pb.h:3054:26: note: candidate function not viable: requires 2 arguments, but 1 was provided
inline void StringTable::add_s(const void* value, size_t size) {

Solution

Problem solved.

The existing StringTable isn't mutable by default. However, using the mutable_ accessors makes it so:

pb.mutable_stringtable().add_s(replace_key);