Tensorflow C++

class member functions C++ API

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TensorFlow's C++ API provides mechanisms for constructing and executing a data flow graph. The API is designed to be simple and concise: graph operations are clearly expressed using a "functional" construction style, including easy specification of names, device placement, etc., and the resulting graph can be efficiently run and the desired outputs fetched in a few lines of code. This guide explains the basic concepts and data structures needed to get started with TensorFlow graph construction and execution in C++.

The Basics

Let's start with a simple example that illustrates graph construction and execution using the C++ API.

// tensorflow/cc/example/example.cc

#include "tensorflow/cc/client/client_session.h"
#include "tensorflow/cc/ops/standard_ops.h"
#include "tensorflow/core/framework/tensor.h"

int main() {
  using namespace tensorflow;
  using namespace tensorflow::ops;
  Scope root = Scope::NewRootScope();
  // Matrix A = [3 2; -1 0]
  auto A = Const(root, { {3.f, 2.f}, {-1.f, 0.f} });
  // Vector b = [3 5]
  auto b = Const(root, { {3.f, 5.f} });
  // v = Ab^T
  auto v = MatMul(root.WithOpName("v"), A, b, MatMul::TransposeB(true));
  std::vector<Tensor> outputs;
  ClientSession session(root);
  // Run and fetch v
  TF_CHECK_OK(session.Run({v}, &outputs));
  // Expect outputs[0] == [19; -3]
  LOG(INFO) << outputs[0].matrix<float>();
  return 0;
}

load("//tensorflow:tensorflow.bzl", "tf_cc_binary")

tf_cc_binary(
    name = "example",
    srcs = ["example.cc"],
    deps = [
        "//tensorflow/cc:cc_ops",
        "//tensorflow/cc:client_session",
        "//tensorflow/core:tensorflow",
    ],
)

Use tf_cc_binary rather than Bazel's native cc_binary to link in necessary symbols from libtensorflow_framework.so. You should be able to build and run the example using the following command (be sure to run ./configure in your build sandbox first):

bazel run -c opt //tensorflow/cc/example:example

This example shows some of the important features of the C++ API, such as the following:

Constructing tensor constants from C++ nested initializer lists
Constructing and naming of TensorFlow operations
Specifying optional attributes to operation constructors
Executing and fetching the tensor values from the TensorFlow session.

We will delve into the details of each below.

Graph Construction

Scope

The Scope object returned by Scope::NewRootScope is referred to as the root scope. "Child" scopes can be constructed from the root scope by calling various member functions of the Scope class, thus forming a hierarchy of scopes. A child scope inherits all of the properties of the parent scope and typically has one property added or changed. For instance, NewSubScope(name)appends name to the prefix of names for operations created using the returnedScope object.

Here are some of the properties controlled by a Scope object:

Operation names
Set of control dependencies for an operation
Device placement for an operation
Kernel attribute for an operation

Operation Constructors

You can create graph operations with operation constructors, one C++ class per TensorFlow operation. Unlike the Python API, which uses snake-case to name the operation constructors, the C++ API uses camel-case to conform to the C++ coding style. For instance, the MatMul theoperation has a C++ class with the same name.

Using this class-per-operation method, it is possible, though not recommended, to construct an operation as follows:

// Not recommended
MatMul m(scope, a, b);

Instead, we recommend the following "functional" style for constructing operations:

// Recommended
auto m = MatMul(scope, a, b);

The first parameter for all operation constructors is always an Scope object. Tensor inputs and mandatory attributes from the rest of the arguments.

For optional arguments, constructors have an optional parameter that allows optional attributes. For operations with optional arguments, the constructor's last optional parameter is a struct type called [operation]:Attrs that contains data members for each optional attribute. You can construct suchAttrs in multiple ways:

You can specify a single optional attribute by constructing an Attrs object using the static functions provided in the C++ class for the operation. For example:

auto m = MatMul(scope, a, b, MatMul::TransposeA(true));

You can specify multiple optional attributes by chaining together functions available in the Attrs struct. For example:

auto m = MatMul(scope, a, b, MatMul::TransposeA(true).TransposeB(true));

// Or, alternatively
auto m = MatMul(scope, a, b, MatMul::Attrs().TransposeA(true).TransposeB(true));

The arguments and return values of operations are handled in different ways depending on their type:

For operations that return single tensors, the object returned by the operation object can be passed directly to other operation constructors. For example:

auto m = MatMul(scope, x, W);
auto sum = Add(scope, m, bias);

For operations producing multiple outputs, the object returned by the operation constructor has a member for each of the outputs. The names of those members are identical to the names present in the OpDef for the operation. For example:

auto u = Unique(scope, a);
// u.y has the unique values and u.idx has the unique indices
auto m = Add(scope, u.y, b);

Operations producing a list-typed output return an object that can be indexed using the [] operator. That object can also be directly passed to other constructors that expect list-typed inputs. For example:

auto s = Split(scope, 0, a, 2);
// Access elements of the returned list.
auto b = Add(scope, s[0], s[1]);
// Pass the list as a whole to other constructors.
auto c = Concat(scope, s, 0);

Constants

Scalars

auto f = Const(scope, 42.0f);
auto s = Const(scope, "hello world!");

Nested initializer lists

// 2x2 matrix
auto c1 = Const(scope, { {1, 2}, {2, 4} });
// 1x3x1 tensor
auto c2 = Const(scope, { { {1}, {2}, {3} } });
// 1x2x0 tensor
auto c3 = ops::Const(scope, { { {}, {} } });

Shapes explicitly specified

// 2x2 matrix with all elements = 10
auto c1 = Const(scope, 10, /* shape */ {2, 2});
// 1x3x2x1 tensor
auto c2 = Const(scope, {1, 2, 3, 4, 5, 6}, /* shape */ {1, 3, 2, 1});

You may directly pass constants to other operation constructors, either by explicitly constructing one using the Const function, or implicitly as any of the above types of C++ values. For example:

// [1 1] * [41; 1]
auto x = MatMul(scope, { {1, 1} }, { {41}, {1} });
// [1 2 3 4] + 10
auto y = Add(scope, {1, 2, 3, 4}, 10);

Graph Execution

Scope root = Scope::NewRootScope();
auto c = Const(root, { {1, 1} });
auto m = MatMul(root, c, { {42}, {1} });

ClientSession session(root);
std::vector<Tensor> outputs;
session.Run({m}, &outputs);
// outputs[0] == {42}

Similarly, the object returned by the operation constructor can be used as the argument to specify a value being fed when executing the graph. Furthermore, the value to feed can be specified with the different kinds of C++ values used to specify tensor constants. For example:

Scope root = Scope::NewRootScope();
auto a = Placeholder(root, DT_INT32);
// [3 3; 3 3]
auto b = Const(root, 3, {2, 2});
auto c = Add(root, a, b);
ClientSession session(root);
std::vector<Tensor> outputs;

// Feed a <- [1 2; 3 4]
session.Run({ {a, { {1, 2}, {3, 4} } } }, {c}, &outputs);
// outputs[0] == [4 5; 6 7]

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