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copied!<p>EDIT: ok the title of your question seems to be misleading</p> <blockquote> <p>"I want a class which takes two parameters in its constructor. The first can be either an int, double or float, so , and the second is always a string literal "my string", so I guess const char * const."</p> </blockquote> <p>It looks like you're trying to achieve:</p> <pre><code>template<typename T> class Foo { public: Foo(T t, const char* s) : first(t), second(s) { // do something } private: T first; const char* second; }; </code></pre> <p>This would work for any type, for the first parameter: <code>int</code>, <code>float</code>, <code>double</code>, whatever.</p> <p>Now if you really want to restrict the type of the first parameter to be only <code>int</code>, <code>float</code> or <code>double</code>; you can come up with something more elaborate like</p> <pre><code>template<typename T> struct RestrictType; template<> struct RestrictType<int> { typedef int Type; }; template<> struct RestrictType<float> { typedef float Type; }; template<> struct RestrictType<double> { typedef double Type; }; template<typename T> class Foo { typedef typename RestrictType<T>::Type FirstType; public: Foo(FirstType t, const char* s) : first(t), second(s) { // do something } private: FirstType first; const char* second; }; int main() { Foo<int> f1(0, "can"); Foo<float> f2(1, "i"); Foo<double> f3(1, "have"); //Foo<char> f4(0, "a pony?"); } </code></pre> <p>If you remove the comment on the last line, you'll effectively get a compiler error.</p> <hr> <p>String literals are not allowed by C++2003</p> <p>ISO/IEC 14882-2003 §14.1:</p> <blockquote> <p><strong>14.1 Template parameters</strong></p> <p>A non-type template-parameter shall have one of the following (optionallycv-qualified) types:</p> <p>— integral or enumeration type, </p> <p>— pointer to object or pointer to function, </p> <p>— reference to object or reference to function, </p> <p>— pointer to member. </p> </blockquote> <p>ISO/IEC 14882-2003 §14.3.2:</p> <blockquote> <p><strong>14.3.2 Template non-type arguments</strong></p> <p>A template-argument for a non-type, non-template template-parameter shall be one of: </p> <p>— an integral constant-expression of integral or enumeration type; or</p> <p>— the name of a non-type template-parameter; or </p> <p>— the address of an object or function with external linkage, including function templates and function template-ids but excluding non-static class members, expressed as & id expression where the & is optional if the name refers to a function or array, or if the corresponding template-parameter is a reference; or</p> <p>— a pointer to member expressed as described in 5.3.1.</p> <p>[Note:A string literal (2.13.4) does not satisfy the requirements of any of these categories and thus is not an acceptable template-argument.</p> <p>[Example: </p> </blockquote> <pre><code>template<class T, char* p> class X { //... X(); X(const char* q) { /* ... */ } }; X<int,"Studebaker"> x1; //error: string literal as template-argument char p[] = "Vivisectionist"; X<int,p> x2; //OK </code></pre> <blockquote> <p>—end example] —end note] </p> </blockquote> <p>And it looks like it's not going to change in the upcoming C++0X, <a href="http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2009/n3000.pdf" rel="nofollow noreferrer">see the current draft 14.4.2 Template non-type arguments</a>.</p>

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