This is originally from Sara's blog.

It also works with almost any version of Visual Studio, you just need to change the "8.0" in the registry key to the appropriate version number for your version of Visual Studio.

The guide line shows up in the Output window too. (Visual Studio 2010 corrects this, and the line only shows up in the code editor window.)

You can also have the guide in multiple columns by listing more than one number after the color specifier:

RGB(230,230,230), 4, 80

Puts a white line at column 4 and column 80. This should be the value of a string value Guides in "Text Editor" key (see bellow).

Be sure to pick a line color that will be visisble on your background. This color won't show up on the default background color in VS. This is the value for a light grey: RGB(221, 221, 221).

Here are the registry keys that I know of:

Visual Studio 2010: HKCU\Software\Microsoft\VisualStudio\10.0\Text Editor

Visual Studio 2008: HKCU\Software\Microsoft\VisualStudio\9.0\Text Editor

Visual Studio 2005: HKCU\Software\Microsoft\VisualStudio\8.0\Text Editor

Visual Studio 2003: HKCU\Software\Microsoft\VisualStudio\7.1\Text Editor

For those running Visual Studio 2010, you may want to install the following extensions rather than changing the registry yourself:

• http://visualstudiogallery.msdn.microsoft.com/en-us/0fbf2878-e678-4577-9fdb-9030389b338c

• http://visualstudiogallery.msdn.microsoft.com/en-us/7f2a6727-2993-4c1d-8f58-ae24df14ea91

These are also part of the Productivity Power Tools, which includes many other very useful extensions.

Caveat: It is not necessary to put the implementation in the header file, see the alternative solution at the end of this answer.

Anyway, the reason your code is failing is that, when instantiating a template, the compiler creates a new class with the given template argument. For example:

template<typename T>
struct Foo
{
    T bar;
    void doSomething(T param) {/* do stuff using T */}
};

// somewhere in a .cpp
Foo<int> f; 

When reading this line, the compiler will create a new class (let's call it FooInt), which is equivalent to the following:

struct FooInt
{
    int bar;
    void doSomething(int param) {/* do stuff using int */}
}

Consequently, the compiler needs to have access to the implementation of the methods, to instantiate them with the template argument (in this case int). If these implementations were not in the header, they wouldn't be accessible, and therefore the compiler wouldn't be able to instantiate the template.

A common solution to this is to write the template declaration in a header file, then implement the class in an implementation file (for example .tpp), and include this implementation file at the end of the header.

Foo.h

template <typename T>
struct Foo
{
    void doSomething(T param);
};

#include "Foo.tpp"

Foo.tpp

template <typename T>
void Foo<T>::doSomething(T param)
{
    //implementation
}

This way, implementation is still separated from declaration, but is accessible to the compiler.

Alternative solution

Another solution is to keep the implementation separated, and explicitly instantiate all the template instances you'll need:

Foo.h

// no implementation
template <typename T> struct Foo { ... };

Foo.cpp

// implementation of Foo's methods

// explicit instantiations
template class Foo<int>;
template class Foo<float>;
// You will only be able to use Foo with int or float

If my explanation isn't clear enough, you can have a look at the C++ Super-FAQ on this subject.