C++: "expected ;" in declaration in template -

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i've been running following problem inside member function of templated class:

#include <map> using std::map; template <typename a,typename b> class c {   public:     b f(const a&,const b&) const;   private:     map<a,b> d; }; template <typename a,typename b> b c<a,b>::f(const a&a,const b&b) const {    map<a,b>::const_iterator x = d.find(a);    if(x == d.end())      return b;    else      return x->second; }

when have g++ compile following error:

bug.c: in member function 'b c<a,b>::f(const a&, const b&) const': bug.c:12: error:expected ';' before 'x' bug.c:13: error: 'x' not declared in scope

however, when make non-templated version of class , function, , b both being int compiles without problem. error little mystifying since can't imagine why wants ';' before 'x'.

you're missing typename:

typename map<a,b>::const_iterator x = d.find(a);

please read where , why have put “template” , “typename” keywords?. reason need typename here because a , b template parameters means meaning of ::const_iterator depends on a , b are. while human name const_iterator makes obvious iterator type, compiler doesn't know if type, data member, etc.

the compiler syntax check on first pass before templates instantiated , adding typename letting compiler know parse map<a,b>::const_iterator type.

also, there special rule in c++ (shamefully stolen linked question):

a name used in template declaration or definition , dependent on template-parameter assumed not name type unless applicable name lookup finds type name or name qualified keyword typename.

if not add typename, compiler has assume not type.


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