Reputation: 970
I was going through a tutorial for building an AST with the help of Clang/LLVM.
I see this syntax there:
struct PPContext {
// Takes ownership of client.
PPContext(clang::DiagnosticClient* client = 0,
const std::string& triple = LLVM_HOSTTRIPLE)
: diagClient(client == 0?new clang::TextDiagnosticPrinter:client),
diags(diagClient),
target(clang::TargetInfo::CreateTargetInfo(triple)),
headers(fm),
pp(diags, opts, *target, sm, headers)
{
// Configure warnings to be similar to what command-line `clang` outputs
// (see tut03).
// XXX: move warning initialization to libDriver
using namespace clang;
diags.setDiagnosticMapping(diag::kind,diag::MAP_IGNORE);
//diag::warn_pp_undef_identifier was the initial value instead of diag::kind. But I changed since it gave error.
}
~PPContext()
{
delete diagClient;
delete target;
}
clang::DiagnosticClient* diagClient;
clang::Diagnostic diags;
clang::LangOptions opts;
clang::TargetInfo* target;
clang::SourceManager sm;
clang::FileManager fm;
clang::HeaderSearch headers;
clang::Preprocessor pp;
};
And:
//What is the constructor doing here ? The construct looks very different and difficult to comprehend !!!
// Could someone break it up for me ?
PPContext(clang::DiagnosticClient* client = 0,
const std::string& triple = LLVM_HOSTTRIPLE)
: diagClient(client == 0?new clang::TextDiagnosticPrinter:client),
diags(diagClient),
target(clang::TargetInfo::CreateTargetInfo(triple)),
headers(fm),
pp(diags, opts, *target, sm, headers)
{
// Configure warnings to be similar to what command-line `clang` outputs
// (see tut03).
// XXX: move warning initialization to libDriver
using namespace clang;
diags.setDiagnosticMapping(diag::kind,diag::MAP_IGNORE);
//diag::warn_pp_undef_identifier was the initial value instead of diag::kind. But I changed since it gave error.
}
Please let me know if there is any other material which would be of good help and learning experience. Thanks
Upvotes: 2
Views: 290
Reputation: 5404
These are constructor initializers. They initialized class members with the value given to them. For instance:
class TestClass
{
private:
int someField;
public:
TestClass() : someField(5) { }
};
Will initialize the member someField to have a value of 5 during the call to the TestClass() constructor. You can separate multiple initializers with a ,
to initialize multiple members. You can also pass parameters from the constructor to these initializers such as:
class TestClass
{
private:
int someField;
public:
TestClass(int _someField) : someField(_someField) { }
};
When this constructor is called, the value passed to _someField will be used to initialize someField.
Also consider inheritance. Using the second TestClass
as our base we get the following derived type:
class TestClassDerived : public TestClass
{
public:
TestClassDerived(int _someField) : TestClass(_someField) { }
};
That's how you can construct a base class from a derived type and pass parameters to a non-default constructor. Without this you wouldn't be able to construct the base with the appropriate parameters using a non-default constructor.
Upvotes: 2
Reputation: 20332
It's called a constructor initialization list, which is used for inheritance and initializating member variables. Click this link for a good explanation of this topic.
One scenario this is required is dealing with inheritance. Consider the following:
class Base
{
public:
Base(int n)
{
}
};
class Derived : Base
{
public:
// Error! How do we construct Base? We never pass n
// to the constructor of Base.
Derived(int n)
{
}
};
How can we change the above to compile? With a constructor initialization list:
class Base
{
public:
Base(int n)
{
}
};
class Derived : Base
{
public:
// Now your compiler is happy.
Derived(int n) : Base(n)
{
}
};
Upvotes: 1
Reputation: 1161
This constructur is in the format
PPContext(arg1, arg2)
: initConstructor1,
...,
initConstructor5
where you can exchange initConstructor to the initialization constructors of the PPContext members
Upvotes: 0