Add var->value() feature

src/tape.cc

@@ -137,14 +137,14 @@ class ScopeWrapper : public framework::Scope {
     for (auto &v : in_vars) {
       for (auto &vv : v.second) {
         if (!vars_.count(vv->Name())) {
-          vars_[vv->Name()].reset(vv->Var());
+          vars_[vv->Name()].reset(vv->MutableVar());
         }
       }
     }
     for (auto &v : out_vars) {
       for (auto &vv : v.second) {
         if (!vars_.count(vv->Name())) {
-          vars_[vv->Name()].reset(vv->Var());
+          vars_[vv->Name()].reset(vv->MutableVar());
         }
       }
     }

src/test_tape.cc

@@ -46,6 +46,7 @@ TEST(Tape, TestMLP) {
     filler(input);
 
     auto loss = mean(linear2(linear1(input)));
+    LOG(INFO) << loss->value();
 
     get_global_tape().Backward(loss);
 

src/variable.cc

@@ -14,9 +14,22 @@
 
 #include "src/variable.h"
 
+#include "src/tape.h"
+
 namespace paddle {
 namespace tape {
 
+std::ostream& operator<<(std::ostream& os, const Variable& var) {
+  LOG(INFO) << "Printing " << var.Name();
+  framework::proto::VarType::Type var_type = var.Desc().GetType();
+  if (var_type == framework::proto::VarType::LOD_TENSOR) {
+    os << var.Var().Get<framework::LoDTensor>();
+  } else {
+    PADDLE_THROW("Variable type is not LOD_TENSOR");
+  }
+  return os;
+}
+
 void Variable::InitializeVariable() {
   LOG(INFO) << "Initialzing " << desc_.Name() << " as " << desc_.GetType();
   framework::proto::VarType::Type var_type = desc_.GetType();
@@ -30,5 +43,10 @@ void Variable::InitializeVariable() {
   }
 }
 
+const Variable& Variable::value() {
+  get_global_tape().Forward();
+  return *this;
+}
+
 }  // namespace tape
 }  // namespace paddle

src/variable.h

@@ -26,6 +26,8 @@ namespace tape {
 class Variable;
 using VariableHandle = std::shared_ptr<Variable>;
 
+std::ostream& operator<<(std::ostream&, const Variable&);
+
 /*
  * Combination of
  *     framework::VarDesc desc_;
@@ -61,15 +63,17 @@ class Variable {
   //  void init(const std::string& initializer,
   //            const framework::AttributeMap& attrs);
 
-  // void value() {};
+  // Evaluate a variable by running Forward() on the global tape
+  const Variable& value();
 
-  const framework::VarDesc &Desc() const { return desc_; }
-  framework::VarDesc *MutableDesc() { return &desc_; }
+  const framework::VarDesc& Desc() const { return desc_; }
+  framework::VarDesc* MutableDesc() { return &desc_; }
 
   // TODO(tonyyang-svail): No need to expose name
   std::string Name() const { return desc_.Name(); }
 
-  framework::Variable *Var() { return &var_; }
+  const framework::Variable& Var() const { return var_; }
+  framework::Variable* MutableVar() { return &var_; }
 
  private:
   int count() {