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    <p>Partial specialization of a function template, whether it is member function template or stand-alone function template, is not allowed by the Standard:</p> <pre><code>template&lt;typename T, typename U&gt; void f() {} //okay - primary template template&lt;typename T&gt; void f&lt;T,int&gt;() {} //error - partial specialization template&lt;&gt; void f&lt;unsigned char,int&gt;() {} //okay - full specialization </code></pre> <p>But you can partially specialize the class template itself. You can do something like this:</p> <pre><code>template &lt;class A&gt; class Thing&lt;A,int&gt; //partial specialization of the class template { //.. int doSomething(); }; template &lt;class A&gt; int Thing&lt;A,int&gt;::doSomething() { /* do whatever you want to do here */ } </code></pre> <p>Note that when you partially specialize a class template, then the template parameter-list of member function (in its definition outside the class), <strong>must match</strong> the template parameter list of the class template partial specialization. That means, for the above partial specialization of the class template, you cannot define this:</p> <pre><code>template &lt;class A&gt; int Thing&lt;A,double&gt;::doSomething(); //error </code></pre> <p>Its not allowed, because the template parameter-list in function definition didn't match the template parameter-list of the class template partial specialization. §14.5.4.3/1 from the Standard (2003) says,</p> <blockquote> <p>The template parameter list of a member of a class template partial specialization <strong>shall match</strong> the template parameter list of the class template partial specialization.[...]</p> </blockquote> <p>For more on this, read my answer here:</p> <p><a href="https://stackoverflow.com/questions/5206080/c-overload-templated-class-method-with-a-partial-specilization-of-that-method/5206117#5206117">C++ - Overload templated class method with a partial specilization of that method</a></p> <hr> <p>So what is the solution? Would you partially specialize your class along with all the repetitive work?</p> <p>A simple solution would be work delegation, instead of partially specializing the class template. Write a <em>stand-alone</em> function template and specialize this as:</p> <pre><code>template &lt;class B&gt; B doTheActualSomething(B &amp; b) { return b; } template &lt;&gt; int doTheActualSomething&lt;int&gt;(int &amp; b) { return b + 1; } </code></pre> <p>And then call this function template from <code>doSomething()</code> member function as:</p> <pre><code>template &lt;class A, class B&gt; B Thing&lt;A,B&gt;::doSomething() { return doTheActualSomething&lt;B&gt;(b_); } </code></pre> <hr> <p>Since in your particular case, <code>doTheActualSomething</code> needs to know the value of <em>only one</em> member, namely <code>b_</code>, the above solution is fine, as you can pass the value to the function as argument whose type is the template <em>type</em> argument <code>B</code>, and specialization for <code>int</code> is possible being it full-specialization.</p> <p>But imagine if it needs to access multiple members, <em>type</em> of each depends on the template <em>type</em> argument-list, then defining a stand-alone function template wouldn't solve the problem, because now there will be more than one <em>type</em> argument to the function template, and you cannot <em>partially</em> specialize the function for just, say, one <em>type</em> (as its not allowed).</p> <p>So in this case you can define a class template instead, which defines a static non-template member function <code>doTheActualSomething</code>. Here is how:</p> <pre><code>template&lt;typename A, typename B&gt; struct Worker { B doTheActualSomething(Thing&lt;A,B&gt; *thing) { return thing-&gt;b_; } }; //partial specialization of the class template itself, for B = int template&lt;typename A&gt; struct Worker&lt;A,int&gt; { int doTheActualSomething(Thing&lt;A,int&gt; *thing) { return thing-&gt;b_ + 1; } }; </code></pre> <p>Notice that you can use <code>thing</code> pointer to access any member of the class. Of course, if it needs to access private members, then you've to make <code>struct Worker</code> a friend of <code>Thing</code> class template, as:</p> <pre><code>//forward class template declaration template&lt;typename T, typename U&gt; struct Worker template &lt;class A, class B&gt; class Thing { template&lt;typename T, typename U&gt; friend struct Worker; //make it friend //... }; </code></pre> <p>Now delegate the work to the friend as:</p> <pre><code>template &lt;class A, class B&gt; B Thing&lt;A,B&gt;::doSomething() { return Worker&lt;A,B&gt;::doTheActualSomething(this); //delegate work } </code></pre> <p>Two points to be noted here:</p> <ul> <li>In this solution, <code>doTheActualSomething</code> is not a member function <em>template</em>. Its not enclosing class which is template. Hence we can <em>partially</em> specialize the class template anytime, to get the desired effect of the <em>partial</em> member function template specialization.</li> <li>Since we pass <code>this</code> pointer as argument to the function, we can access any member of the class <code>Thing&lt;A,B&gt;</code>, even private members, as <code>Worker&lt;T,U&gt;</code> is also a friend.</li> </ul> <p>Complete online demo : <a href="http://www.ideone.com/uEQ4S" rel="noreferrer">http://www.ideone.com/uEQ4S</a></p> <hr> <p>Now there is still a chance of improvement. Now all instantiations of <code>Worker</code> class template are friends of all instantiation of <code>Thing</code> class template. So we can restrict this many-to-many friendship as:</p> <pre><code>template &lt;class A, class B&gt; class Thing { friend struct Worker&lt;A,B&gt;; //make it friend //... }; </code></pre> <p>Now only one instantiation of <code>Worker</code> class template is a friend of one instantiation of <code>Thing</code> class template. That is one-to-one friendship. That is, <code>Worker&lt;A,B&gt;</code> is a friend of <code>Thing&lt;A,B&gt;</code>. <code>Worker&lt;A,B&gt;</code> is NOT a friend of <code>Thing&lt;A,C&gt;</code>. </p> <p>This change requires us to write the code in somewhat different order. See the complete demo, with all the ordering of class and function definitions and all: </p> <p><a href="http://www.ideone.com/6a1Ih" rel="noreferrer">http://www.ideone.com/6a1Ih</a></p>
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    1. COGood link. I read in your quotation "A class template specialization is a distinct template.". Does this mean that my class specialization will have to define all the expected member data of the original `Thing<A,B>`? As I read it, the answer is yes...
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    2. COAn interesting bypass of the problem of having to redefine everything is also listed in the link you sent... see my use of `foo` above, as adapted from Alf P. Steinbach's answer.
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    3. COThe answer is wrong: *Specialization of member function without specializing the class template, is not allowed* is **false**. You cannot *partially* specialize a member function in any context but you can specialize a member function of a class template. `template <typename T> struct test { void foo() {} }; template <> void test<int>::foo() { std::cout << "specialized"; }` is perfectly valid.
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      1. USDavid Rodríguez - dribeas
 

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