How can I create a template instance which has a char* as argument?
I tried:
template <const char* const N1,typename T1>
class name_value_pair{
T1& val;
name_value_pair(T1& valx) : val(valx) {}
print(){
cout << N1 << "=" << val << endl;
}
};
int main(){
string alpha="123";
name_value_pair<"alpha",ty
x.print();
return 0;
}
...but this results in (g++):
string_templates.cpp:18: error: string literal "alpha" is not a valid template argument because it is the address of an object with static linkage
Using const char[] instead doesn't work - same error.
Ideas, anyone?
Stefan
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This is what the C++ Standard says
A template argument for a non-type, non-template template-parameter shall be on of
- an integral const expr of integral or enum type
- name of non-type template parameter
- name of object or function with external linkage
- addr of object of function with external linkae
A string literal is not acceptable template-argument because a string literal has internal linkage
Amit
chance7,
OK, it's a matter of visibility and encapsulation. But as this URL says, once something is in a name space, it's external. So I did:
namespace weird{
const char xx[]="alpha";
extern const char xx[];
}
int main(){
string alpha="123";
name_value_pair<weird::xx,
x.print();
return 0;
}
...but still,
string_templates.cpp:23: error: address of non-extern `weird::xx' cannot be used as template argument
This doesn't work. However, this works:
namespace weird{
extern const char xx[];
}
int main(){
string alpha="123";
name_value_pair<weird::xx,
x.print();
return 0;
}
namespace weird{
const char xx[]="alpha";
}
I start wordering if g++ has a glitch here in implementing the standard...
Stefan
I don't have time to test this, but is the problem actually that the type of the actual argument (decays to) char*, and the type of the formal parameter is const char* const ? You might get this to work either by changing the type of the template formal parameter to char* or by static_cast'ing the actual argument to ((const char* const)"xyz")
I think this is directly about C style string constants default to internal linkage and thus are not allowed as template parameters by C++ standard.
However, stefan73, I got curious and found out, that namespace by itself did not matter at all. Even simply defining xx as:
extern const char xx[]="alpha";
in global scope worked for g++ at least.
I guess your first attempt failed because of the order you wrote things. g++ did not like you defined xx first and then declared it external, therefore probably ignoring the external declaration entirely having given xx an internal linkage already.
For the real reason I wish someone with deeper knowledge of compiler anatomy could throw in a comment about this.
Regards,
chance
chance7,
> extern const char xx[]="alpha";
Yes, you're right. This works. Only if you try to put this into local scope, you get an error message which would be appropriate for a global or anonymous namespace:
string_templates.cpp:22: error: `xx' has both `extern' and initializer
Strange.
What I was trying to do is to have a template as a singleton for each function or method with __LINE__ and __FILE__ as parameters (so you can store something as const in it + effectively have nested functions). Apparently, for this the __FILE__ cannot be used directly (via macro, that is). A pity.
I still wonder, though, why a template needs to be instantiated with external linkage only. For the purpose I use it for, I wouldn't really care.
Stefan
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Answer for Membership
by: chance7Posted on 2004-05-13 at 02:51:04ID: 11057517
That is not allowed. Read the reasons and examples from the link below.
http://www.comeaucomputing
Regards,
chance