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POHow to use a class Y defined later in the code inside class X defined earlier?
 

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  1. POHow to use a class Y defined later in the code inside class X defined earlier?
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    <p>I have this code and it compiles successfully.</p> <pre><code>#include &lt;iostream&gt; #include &lt;memory&gt; template &lt;typename T&gt; class Expression { public: T _data; Expression() {} Expression(T data) { _data = data; } void foo(); }; template &lt;typename T&gt; class ConstExpression : public Expression&lt;T&gt; { public: ConstExpression() {} ConstExpression(T data) { this-&gt;_data = data; } }; template &lt;typename T&gt; void Expression&lt;T&gt;::foo() { std::shared_ptr&lt;ConstExpression&lt;T&gt;&gt; temp(new Expression); std::shared_ptr&lt;ConstExpression&lt;T&gt;&gt; temp2(new ConstExpression&lt;T&gt;()); ConstExpression&lt;T&gt; foo2(5); } int main() { return 0; } </code></pre> <p>Now if I do the following or give my function foo an input argument of type constexpression I get an error: main.cc:15:25: error: use of undeclared identifier 'ConstExpression'; did you mean 'Expression'? In my real code my function takes an input argument of type ConstExpression and I have done an out of line declaration too but get a similar error.</p> <pre><code>#include &lt;iostream&gt; #include &lt;memory&gt; template &lt;typename T&gt; class Expression { public: T _data; Expression() {} Expression(T data) { _data = data; } void foo() { std::shared_ptr&lt;ConstExpression&lt;T&gt;&gt; temp(new Expression); std::shared_ptr&lt;ConstExpression&lt;T&gt;&gt; temp2(new ConstExpression&lt;T&gt;()); ConstExpression&lt;T&gt; foo2(5); } }; template &lt;typename T&gt; class ConstExpression : public Expression&lt;T&gt; { public: ConstExpression() {} ConstExpression(T data) { this-&gt;_data = data; } }; int main() { return 0; } </code></pre>
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    1. USpizzaEatingGuy
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    1. COYou need to use forward declaration. Have a look at this post http://stackoverflow.com/questions/553682/when-to-use-forward-declaration
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      1. USSantanu C
    2. COThis: `std::shared_ptr<ConstExpression<T>> temp(new Expression);` wouldn't be legal even if the other problems are solved. `Expression` is a template and you're providing no template parameter, and even if you were, it isn't a derivation of `ConstExpression<T>`; rather its the other way around.
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      1. POHow to use a class Y defined later in the code inside class X defined earlier?
      1. USWhozCraig
    3. COThe REAL problem (besides the aforementioned problems) is that you have a base class that knows about a derived class. It seems like inheritance isn't the right solution to the problem you are trying to solve.
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      1. USKSletmoe
 

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