Exporting Templated Classes from a DLL Without Explicit Specification
When working with DLLs (Dynamic Link Libraries) in C++, one often encounters the need to export templated classes from the DLL. This can be a challenging task, as the compiler needs explicit specification of the template parameters to generate the necessary code. However, there are ways to export templated classes without explicitly specifying the template parameters, and in this article, we will explore one such method.
Before we dive into the solution, let's first understand why exporting templated classes from a DLL can be a problem. When we declare a templated class, the compiler generates code for the class based on the template parameters we specify. This code is then used to instantiate the class with different data types at compile time. However, when we try to export this code from a DLL, the compiler needs to know the template parameters at compile time, which is not possible as the DLL is compiled separately from the client code.
To overcome this problem, we can use a technique called explicit instantiation. This involves explicitly specifying the template parameters for the DLL, allowing the compiler to generate the necessary code. However, this can be a tedious and error-prone task, especially when working with complex templated classes.
To avoid the hassle of explicit instantiation, we can use an alternative approach known as the "export" keyword. This keyword was introduced in C++11 and allows us to export templated classes without explicitly specifying the template parameters. Let's see how it works.
First, we need to declare our templated class with the "export" keyword before the class declaration. For example:
export template <typename T>
class MyClass {
// class definition
};
Next, we need to use the "export" keyword again when defining the class in the DLL. This will tell the compiler to export the class from the DLL without explicitly instantiating it. For example:
export template <typename T>
MyClass<T>::MyClass() {
// constructor code
}
By using the "export" keyword, we are telling the compiler to generate the necessary code for the class during the linking process. This means that the client code does not need to explicitly specify the template parameters, as the code for the class will be available in the DLL.
One important thing to note is that the "export" keyword is only supported by certain compilers, such as Microsoft Visual C++. Other compilers may have their own syntax for exporting templated classes, so it's important to check the documentation for your specific compiler.
In addition to the "export" keyword, there is another way to export templated classes without explicitly specifying the template parameters, and that is by using a "dummy" type parameter. This involves adding an extra, unused type parameter to the class declaration and definition. For example:
template <typename T, typename = void>
class MyClass {
// class definition
};
template <typename T, typename>
MyClass<T>::MyClass() {
// constructor code
}
This "dummy" type parameter allows the compiler to generate the necessary code for the class without explicitly specifying the template parameters. This approach may be preferred by some developers as it is more widely supported by different compilers.
In conclusion, exporting templated classes from a DLL without explicitly specifying the template parameters can be achieved by using the "export" keyword or a "dummy" type parameter. These methods allow us to avoid the hassle of explicit instantiation and make our code more portable. However
