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问题描述

在BGL中，add_edge检查边是否已存在. add_vertex是否有相同的机制?例如，如果我们有一个包含某些bundled_properties的顶点，然后将其添加到图形中，然后将其连接到一条边，那么如果存在相似的顶点，该顶点是否会被复制?

In BGL, add_edge checks if the edge already exists. Is there the same mechanism for add_vertex?For example, if we have a vertex with some bundled_properties, and then we add it to the graph, and then connect an edge to it, would be this vertex duplicated if there was a similar vertex?

让我们看一下`add_vertex`:

调用为add_vertex，它添加了一个顶点.实际上，通常不为它们插入属性，这只是一种方便.

Let's look at `add_vertex`:

The call is add_vertex and it adds a vertex. In fact, you don't usually insert properties for them, that's merely a convenience.

之间没有根本区别:

vertex_descriptor v = add_vertex(g);

g[v] = vertexProperties;

还有

vertex_descriptor v = add_vertex(vertexProperties, g);

在两种情况下，您都会总是获得一个新的，唯一的顶点描述符，并将其属性设置为特定值.

In both cases you will always get a new, unique, vertex descriptor, and have its properties set to a specific value.

add_edge真的不同吗?与add_vertex一样，两者之间没有区别:

Is add_edge really different? Like with add_vertex there is no difference between:

vertex_descriptor from {/*...*/}, to {/*...*/};
edge_descriptor e = add_edge(from, to, g).first;

g[e] = edgeProperties;

还有

edge_descriptor e = add_edge(from, to, edgeProperties, g).first;

您会注意到，两者(可能)都添加了一条边，返回其描述符，并且都将属性设置为特定值.

You will note that both (possibly) add an edge, returning its descriptor, and both set the properties to specific values.

为什么要`add_edge` 有条件地添加?

这是因为adjacency_list使用的策略隐含需要检查的不变性:

Why does `add_edge` conditionally add?

That's because the adjacency_list uses strategies that imply invariants that need to be checked:

如果您的边缘容器选择恰好是setS，其插入方法可能会或可能不会插入新边； adjacency_lists<>只是将行为¹转发给add_edge
如果您的adjacency_list也使用undirected或bidirectional，则会在边上放置更多约束(例如，防止在使用setS作为目标的双向图中添加(a->b)和(b->a))边缘容器选择器).

if your edge-container selection happens to be e.g. setS, its insertion method may or may not insert a new edge; adjacency_lists<> just forwards the behaviour¹ to add_edge
if your adjacency_list uses undirected or bidirectional as well, this puts extra contraints on edges (e.g. preventing addition of (a->b) as well as (b->a) in a bidirectional graph that uses setS as the edge container selector).

总结:

add_edge 确实获取标识信息(端点顶点)，并且可以对照实例化adjacency_list时选择的策略所产生的约束来检查这些信息.

add_edge does take identifying information (the endpoint vertices) and it can check these against the constraints that arise from the strategies that you chose when instantiating the adjacency_list.

¹例如优雅地避免边缘加倍

¹ e.g. to elegantly avoid doubled edges

²或其他可能的随机访问顶点容器选择器，它们具有作为隐式顶点ID

² or possibly other Random Access vertex container selectors, that have integral vertex descriptors that act as implicit vertex id

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如果BGL中的add_vertex检查顶点的存在

问题描述

推荐答案

让我们看一下add_vertex:

Let's look at add_vertex:

为什么要add_edge 有条件地添加?

Why does add_edge conditionally add?

让我们看一下`add_vertex`:

Let's look at `add_vertex`:

为什么要`add_edge` 有条件地添加?

Why does `add_edge` conditionally add?