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[TOC]
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++.
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;
}
Place this example code in the file tensorflow/cc/example/example.cc
inside a
clone of the
TensorFlow
github repository. Also place a
BUILD
file in the same directory with the following contents:
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.
tensorflow::Scope
is the main data structure that holds the current state
of graph construction. A Scope
acts as a handle to the graph being
constructed, as well as storing TensorFlow operation properties. The Scope
object is the first argument to operation constructors, and operations that use
a given Scope
as their first argument inherit that Scope
's properties, such
as a common name prefix. Multiple Scope
s can refer to the same graph, as
explained further below.
Create a new Scope
object by calling Scope::NewRootScope
. This creates
some resources such as a graph to which operations are added. It also creates a
tensorflow::Status
object which will be used to indicate errors encountered
when constructing operations. The Scope
class has value semantics, thus, a
Scope
object can be freely copied and passed around.
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 returned
Scope
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
Please refer to tensorflow::Scope
for the complete list of member functions
that let you create child scopes with new properties.
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 C++ coding
style. For instance, the MatMul
operation 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 a Scope
object.
Tensor inputs and mandatory attributes form 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 such
Attrs
in multiple ways:
- You can specify a single optional attribute by constructing an
Attrs
object using thestatic
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);
You may pass many different types of C++ values directly to tensor
constants. You may explicitly create a tensor constant by calling the
tensorflow::ops::Const
function from various kinds of C++ values. For
example:
- 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);
When executing a graph, you will need a session. The C++ API provides a
tensorflow::ClientSession
class that will execute ops created by the
operation constructors. TensorFlow will automatically determine which parts of
the graph need to be executed, and what values need feeding. For example:
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]
Please see the tensorflow::Tensor
documentation for more information on how
to use the execution output.