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    <p>C++, like Java, is a statically typed and statically scoped language. This means that the type and lifetime of variables must be known at compilation time.</p> <blockquote> <p>In java I always thought after a class is declared you can call its construtor at any time, but I have no idea how to do that in C++.</p> </blockquote> <p>This is not different from Java. In Java, what you're attempting would be equivalent to doing:</p> <pre><code>MyClass c; if (cond) { MyClass c = new MyClass(1); } else { MyClass c = new MyClass(2); } </code></pre> <p>This has <em>nothing</em> to do with calling constructors. It has to do with the fact that you're <em>declaring</em> new variables in nested scopes, and they are completely independent of variables of the same name in outer scopes.</p> <p>If you need runtime polymorphism, then (by definition), you actually need to use polymorphism. That is, you need to create a common base class with virtual functions:</p> <pre><code>class SimpleVectorBase { public: SimpleVectorBase() { } virtual ~SimpleVectorBase() { } virtual int size() const { return length; } // ... etc. ... private: int length; } template &lt;class T&gt; class SimpleVector : public SimpleVectorBase { // ... } int main() { // ... SimpleVectorBase* list1; if (dType == 0) { list1 = new SimpleVector&lt;int&gt;(dataSize); } else if (dType == 1) { list1 = new SimpleVector&lt;double&gt;(dataSize); } else { list1 = new SimpleVector&lt;string&gt;(dataSize); } // ... } </code></pre> <p>However, doing this won't really help you with your <code>for</code> loop. In your particular case, you probably would be better off templatizing the whole thing:</p> <pre><code>template&lt;typename T&gt; void doWork(int dataSize) { SimpleVector&lt;T&gt; list1(dataSize); std::cout &lt;&lt; "Please enter the data:" &lt;&lt; std::endl; for (int i = 0; i &lt; dataSize; i++) { std::cin &gt;&gt; list1[i]; } // ... Do other stuff with list1 ... } </code></pre> <p>and then your <code>main()</code> function would do:</p> <pre><code>if (dType == 0) { doWork&lt;int&gt;(dataSize); } else if (dType == 1) { doWork&lt;double&gt;(dataSize); } else { doWork&lt;string&gt;(dataSize); } </code></pre>
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    1. POChoosing the data type for a generalized template class in main()
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    1. COI had another question about my 3rd constructor. It is supposed to implement a "deep copy" of the vector parameter. I am trying to simply make a new array with the size of the input parameter array. However the line: template <class T> SimpleVector<T>::SimpleVector(const SimpleVector & copy) { int newSize = copy - > size();//THIS LINE doesn't seem to work correctly
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    2. COThanks guys I've gone ahead and split my if statement to just act on the list depending on the data type chosen. (I'm sure generalizing a read data would have been the best way to go, but whatever repetitive code wont hurt me point wise)
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    3. CO@user1871426: References aren't pointers. You would need to use `copy.size()`, not `copy->size()`. (Also, look into using *initialization lists* for constructors, and "copy" is a misnomer since that argument is what's being copied, not itself a copy.)
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