Retrieve template parameters from CXXConstructExpr in Clang AST

Question

Say I have a variable declaration like this:

std::vector<MyType> myVector(1);

This is represented as a CXXConstructExpr in the Clang AST. I have a matcher that finds this CXXConstructExpr, but I'd like to extract the decl for MyType from it.

I've tried all sorts of things, but nothing seems to work:

const CXXConstructExpr* construct = Result.Nodes.getNodeAs<CXXConstructExpr>("expr");
construct->getConstructor()->getTemplateSpecializationArgs()  // Always nullptr
construct->getConstructor()->getParent()  // Seems to lose the template parameters
construct->getConstructor()->getDescribedTemplate()  // Always nullptr

Some Who Call Me Tim · Accepted Answer · 2017-06-09 04:42:47Z

Here's a matcher:

varDecl(
  has(
    cxxConstructExpr()
  )
 ,hasType(
    classTemplateSpecializationDecl().bind(sp_dcl_bd_name_)
  )
).bind(var_bd_name_);

It starts with the VarDecl and traverses to the type, which is a ClassTemplateSpecializationDecl buried in vector's ClassTemplateDecl. In the callback, one can work from the ClassTemplateSpecializationDecl to the template argument list, and operate on the individual template arguments:

using CTSD = ClassTemplateSpecializationDecl;
CTSD * spec_decl =
    const_cast<CTSD *>(result.Nodes.getNodeAs<CTSD>(sp_dcl_bd_name_));
VarDecl * var_decl =
    const_cast<VarDecl *>(result.Nodes.getNodeAs<VarDecl>(var_bd_name_));
if(spec_decl && var_decl) {
  // get the template args
  TemplateArgumentList const &tal(spec_decl->getTemplateArgs());
  for(unsigned i = 0; i < tal.size(); ++i){
    TemplateArgument const &ta(tal[i]);
    // is this arg a type arg? If so, get that type
    TemplateArgument::ArgKind k(ta.getKind());
    std::string argName = "";
    if(k==TemplateArgument::ArgKind::Type){
      QualType t = ta.getAsType();
      argName = t.getAsString();
    }
    // Could do similar actions for integral args, etc...
    std::cout << "For variable declared at "
      << corct::sourceRangeAsString(var_decl->getSourceRange(),&sm) << ":"
      << spec_decl->getNameAsString()
      << ": template arg " << (i+1) << ": " << argName << std::endl;
  } // for template args
} // if

For this code:

struct B{int b_;};
std::vector<B> vb(1);

this produces:

For variable declared at <line:14:1, col:20>:vector: template arg 1: struct B
For variable declared at <col:1, col:20>:vector: template arg 2: class std::__1::allocator<struct B>

The full example is in the Code Analysis and Refactoring with Clang Tools examples repo at github: https://github.com/lanl/CoARCT (see apps/TemplateType.cc)

I had to use hasDescendant() instead of has() because the CXXConstructExpr isn't always a direct child of the VarDecl, but this otherwise worked.

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