Fix constructor bug in mixed_vector (#8364)

* Fix constructor bug in mixed_vector

* Fix warnings

* Clean code

* Extract for-loop init. Make nvcc happy

Author: reyoung

paddle/fluid/framework/mixed_vector.h

@@ -37,9 +37,8 @@ class Vector {
 
   // Fill vector with value. The vector size is `count`.
   explicit Vector(size_t count, const T& value = T()) {
-    if (count == 0) {
-      InitEmpty();
-    } else {
+    InitEmpty();
+    if (count != 0) {
       resize(count);
       T* ptr = begin();
       for (size_t i = 0; i < count; ++i) {
@@ -122,6 +121,10 @@ class Vector {
   const T* begin() const { return &this->operator[](0); }
   const T* end() const { return &this->operator[](size()); }
 
+  const T* cbegin() const { return begin(); }
+
+  const T* cend() const { return end(); }
+
   const T& back() const {
     auto it = end();
     --it;
@@ -244,7 +247,9 @@ class Vector {
 
   bool operator==(const Vector<T>& other) const {
     if (size() != other.size()) return false;
-    for (auto it1 = begin(), it2 = other.begin(); it1 < end(); ++it1, ++it2) {
+    auto it1 = cbegin();
+    auto it2 = other.cbegin();
+    for (; it1 < cend(); ++it1, ++it2) {
       if (*it1 != *it2) {
         return false;
       }

paddle/fluid/framework/mixed_vector_test.cu

@@ -26,10 +26,10 @@ TEST(mixed_vector, CPU_VECTOR) {
   for (int i = 0; i < 10; ++i) {
     tmp.push_back(i);
   }
-  ASSERT_EQ(tmp.size(), 10);
+  ASSERT_EQ(tmp.size(), 10UL);
   vec<int> tmp2;
   tmp2 = tmp;
-  ASSERT_EQ(tmp2.size(), 10);
+  ASSERT_EQ(tmp2.size(), 10UL);
   for (int i = 0; i < 10; ++i) {
     ASSERT_EQ(tmp2[i], i);
     ASSERT_EQ(tmp2[i], tmp[i]);
@@ -58,7 +58,7 @@ TEST(mixed_vector, GPU_VECTOR) {
   for (int i = 0; i < 10; ++i) {
     tmp.push_back(i);
   }
-  ASSERT_EQ(tmp.size(), 10);
+  ASSERT_EQ(tmp.size(), 10UL);
   paddle::platform::CUDAPlace gpu(0);
 
   multiply_10<<<1, 1, 0, GetCUDAStream(gpu)>>>(tmp.MutableData(gpu));
@@ -79,7 +79,7 @@ TEST(mixed_vector, MultiGPU) {
   for (int i = 0; i < 10; ++i) {
     tmp.push_back(i);
   }
-  ASSERT_EQ(tmp.size(), 10);
+  ASSERT_EQ(tmp.size(), 10UL);
   paddle::platform::CUDAPlace gpu0(0);
   paddle::platform::SetDeviceId(0);
   multiply_10<<<1, 1, 0, GetCUDAStream(gpu0)>>>(tmp.MutableData(gpu0));
@@ -91,3 +91,10 @@ TEST(mixed_vector, MultiGPU) {
     ASSERT_EQ(tmp[i], i * 100);
   }
 }
+
+TEST(mixed_vector, InitWithCount) {
+  paddle::framework::Vector<int> vec(10, 10);
+  for (int i = 0; i < 10; ++i) {
+    ASSERT_EQ(vec[i], 10);
+  }
+}