未使用using namespace at，为何at::zeros仍为函数重载候选？

问题代码 #

#include <iostream>
#include <torch/torch.h>
using namespace torch;
int main() {
    std::cout << zeros({}) << std::endl;
}

编译报错信息 #

% g++ -lc10 -ltorch -ltorch_cpu test.cpp -o test.x                                                        
test.cpp: In function ‘int main()’:
test.cpp:5:23: error: call of overloaded ‘zeros(<brace-enclosed initializer list>)’ is ambiguous
    5 |     std::cout << zeros({}) << std::endl;
      |                  ~~~~~^~~~
In file included from /opt/links/include/ATen/Functions.h:1334,
                 from /opt/links/include/ATen/ExpandUtils.h:4,
                 from /opt/links/include/torch/csrc/autograd/input_metadata.h:3,
                 from /opt/links/include/torch/csrc/autograd/function.h:7,
                 from /opt/links/include/torch/csrc/autograd/custom_function.h:7,
                 from /opt/links/include/torch/csrc/api/include/torch/autograd.h:5,
                 from /opt/links/include/torch/csrc/api/include/torch/all.h:7,
                 from /opt/links/include/torch/csrc/api/include/torch/torch.h:3,
                 from test.cpp:2:
/opt/links/include/ATen/ops/zeros.h:35:19: note: candidate: ‘at::Tensor at::zeros(IntArrayRef, c10::TensorOptions)’
   35 | inline at::Tensor zeros(at::IntArrayRef size, at::TensorOptions options={}) {
      |                   ^~~~~
In file included from /opt/links/include/torch/csrc/api/include/torch/types.h:7,
                 from /opt/links/include/torch/csrc/api/include/torch/data/dataloader_options.h:4,
                 from /opt/links/include/torch/csrc/api/include/torch/data/dataloader/base.h:3,
                 from /opt/links/include/torch/csrc/api/include/torch/data/dataloader/stateful.h:4,
                 from /opt/links/include/torch/csrc/api/include/torch/data/dataloader.h:3,
                 from /opt/links/include/torch/csrc/api/include/torch/data.h:3,
                 from /opt/links/include/torch/csrc/api/include/torch/all.h:9:
/opt/links/include/torch/csrc/autograd/generated/variable_factories.h:565:19: note: candidate: ‘at::Tensor torch::zeros(at::IntArrayRef, c10::TensorOptions)’
  565 | inline at::Tensor zeros(at::IntArrayRef size, at::TensorOptions options = {}) {
      |                   ^~~~~

核心疑问 #

未使用using namespace at，为何at::zeros会被列为重载候选？

原因解析 #

这是C++**依赖参数查找（ADL，又称Koenig查找）**导致的：

• 调用zeros({})时，编译器会将{}推导为at::IntArrayRef（该类型支持用初始化列表构造）。
• 根据ADL规则，编译器会自动在参数类型所属的命名空间（即at）中查找同名函数，因此at::zeros会被纳入重载候选，和因using namespace torch而可见的torch::zeros产生歧义。

解决方法 #

以下几种方式均可消除歧义：

• 明确指定命名空间：直接限定调用torch命名空间下的函数

  std::cout << torch::zeros({}) << std::endl;
  

• 显式构造参数类型：将初始化列表包装为at::IntArrayRef，避免ADL触发对at命名空间的查找

  std::cout << zeros(at::IntArrayRef{}) << std::endl;
  

内容的提问来源于stack exchange，提问作者Daniel Wagner