no member named 'print' in 'std::__1::basic_string<char>'

Question

if I declare a string and attempt to print it like this:

std::string s;
eosio::print(s);

the EOS compiler pukes:

In file included from t.cpp:1: In file included from ./t.hpp:4: In file included from /usr/local/Cellar/eosio.cdt/1.4.1/opt/eosio.cdt/bin/../include/eosiolib/asset.hpp:3: /usr/local/Cellar/eosio.cdt/1.4.1/opt/eosio.cdt/bin/../include/eosiolib/print.hpp:237:9: error: no member named 'print' in 'std::__1::basic_string' t.print(); ~ ^ t.cpp:126:3: note: in instantiation of function template specialization 'eosio::print &>' requested here print(ret); ^ 1 error generated.

but if I convert it:

eosio::print(std::string(s));

it works fine. why is this necessary? s is already a string so why do I need this?

Answer 1

As the compiler indicates

.../include/eosiolib/print.hpp:237:9: error: no member named 'print' in 'std::__1::basic_string' t.print(); ...

after check the the source code of print.hpp

inline void print( const std::string& s) {
  prints_l( s.c_str(), s.size() );
}

inline void print( std::string&& s) {
  prints_l( s.c_str(), s.size() );
}

...

template<typename T>
inline void print( T&& t ) {
   t.print();
}

then you will see that when you use

std::string s;
eosio::print(s);

it matches the print( T&& t) method best, so that will call the string.print which is not exist.

whereas use the

eosio::print(std::string(s));   // or eosio::print(std::move(s));

it matches the print( std::string&& s)